Here is a picture of the skin trimmed.
An old picture showing where the four spars are. I did add two cross braces (no photo) and braces all the way forward at the bottom of where the skin goes. With the skin on I cut the four spars in between the two skins.
A couple of photos showing the hatch propped open.
A seal has to run down both sides, and 1/8" ply is not wide enough. My solution is to glue more ply on the inside. I added three layers, with each layer just a little further back, so I could sand it parallel to the skin that will be put there.
This process took several days, and many many clamps.
Time for the first side of skin.
Trim to shape. I used an 8' aluminum ruler to find the trim line.
Installed the second side and trimmed.
The latch is installed flush with the top of the counter. Trim so it fits through the braces.
The top piece is 3/4" ply trimmed so the base of the latch sticks through. The latch bolts to this piece of ply.
I added these two pieces of 1 by that are only glued to the spars. I used spacers 5.2mm ply to hold the bottom framework off the body (to leave room for seal). The spars in time will be cut from the body (no it is not time yet)
Another view.
In the space left I added a small triangle of 1/2" ply secured to the 1 by, that a metal striker will mount to. Note the small rectangle piece of ply is a temporary shim. 
When I laid out the sides I marked the door cut out, which should show through the fiberglass. Using a jig saw I cut out the window first, then the door.
Now I need to build a lip for the door to seal on. Andrew made this drawing (thanks Andrew). The purple (maybe it is blue) is the body. I am adding two pieces, an 1/8" spacer, to give room for the seal (black in the illustration) and the lip.
Due to the complex shape, I built the spacer out of many small pieces. Each piece is custom fit and goes all the way back to the interior spar. Cut out the general shape and held it in place and drew the curve, cut out with jig saw, glue and clamp. Do not make more than you can clamp. Let dry overnight, remove clamps and sand for exact shape. I made the lip out of two layers of 5.2mm ply (I have no idea where it came from, but it is nice thin ply). The lip is 3/4". Do not put the seams in the same place. This took about a week of clamping, letting the glue dry, cutting a piece to go over it, gluing, clamping.....
After sanding, coat the entire thing with fiberglass resin.
To put a bottom on the tag box I cut a piece of 1/4 ply wide enough to fit between the braces, and slid it in. Holding it in place I traced the shape of the skin and cut.
I glued it in place, set my clamps to spread , and held it in place.
Added some fiberglass resin to coat the top, and filleted the bottom.
I hid the compartment vent in front of the tag (the battery goes in this compartment). When stapling this panel on I clamped the bottom so it bowed upward (figured the water would flow out).
Once glued and stapled, remove metal and fiberglass.
Before getting started make sure no staples are sticking up. Grind and/or pound the offending staples. The hardest part is getting the cloth to lay flat. Some one on the forum suggested using tape, to whom ever suggested it thank you, I have CRS and can not remember who.
You end up stretching the cloth slightly to get the wrinkles out. Focus your attention on the panel you are working on. I leave the overlap, and trim out the big folds. Yes when I work on the next panel it overlapped this one back the other way. The corners are covered twice.
To apply the resin I use a paint roller. I bought a half size roller handle, and cut full size rolers in half to fit it (thanks Doug). Use a paint roller pan with inserts to mix the resin. Start in one place and work away from it pushing bubbles away, remove tape as you go (do not bury it, ok try not to. Any staples sticking up will produce bubbles. The cloth I used is only 5' wide, so I will have to fill the bottom later. If I build this trailer again it will not be as tall, fixing this challenge.
Here is a picture of tape that is buried, shh do not tel anyone.
For the roof I used 1/4" ply. The rear section (already done in picture) required one brace from side to side. Next decide where the vent will go in front. I decided centered, using the existing brace. Build a square 14 1/4" big. Note the directions with the vent say 14" but to do that, double the braces so the mounting screws have something to bite into.
Trial fit of vent.
Glue and staple the front section on. When you cut out the roof make it a little larger than necessary and sand to fit. The hole for the vent I cut out with a router and a trim bit, that is till the bearing on the trim bit burnt up, then I used a jig saw.
Sand the roof to fit, and fill the cracks with fiberglass.
There are several schools of thought on how to seal the bottom of a trailer from water, you will find several threads on the subject on Mikes forum. Here is mine. First I removed the frame (paint it now) and rolled the body onto its side.
I used scraps of 1/8" ply to fill the gaps. When I covered the body mounts, I reached through the large openings and drill through the ply. Before covering the large openings I added a piece of framework. As you work glue in the spacers, clamping them with the body mount bolts.
For the three pieces in the nose I used thicker scrap, and added 1/8" stuff to fill the gaps. 
When done it should look kinda like this. Do not mind the mixed match ply it all gets covered in black goo.
Most states require a tag on the back of the trailer, this is how I did mine. I used the bracket of the harbor freight trailer and put a 90 degree bend in it so it would mount on the bottom of the wood brace (not shown). Pictured is the first of three wooden braces I put here, the other two are sandwiched parallel to this one, going towards the front. Yes the tag is facing the wrong way.
I cut a line above and below it and two on the sides. I hated it.
Here is one corrected line, I think it looks better. Now cut the other side.
Tail lites need to be added to your trailer, how you choose to deal with them is your choice. I choose round ones, frenched in, with blue dots (check with local laws before installing blue dots).
I bought a hole saw that matched the OD of the pipe, and an extra long center bit for the saw. Mark center points and drill the holes in line with the center line of the trailer, not the skin. A lot of fitting and fusing will gain you two tubes that match. Once you are happy with the location, clamp into place and fiberglass in place on the inside.
Inside view of clamps. Let dry.
After resin dries trim the outside flush. I used 1/2 ply for the backing, fiberglassed in, to mount the lites. This method of installing lites will gain you minimal lighting for your trailer, if you desire more or different get creative. On the forum we debated whether I needed more lighting. There were two camps, and I chose to go this way knowing if someone did not notice my small lights, on a bright yellow trailer, they were not paying attention, and would not notice ten time more lights. My 2c.
To put the skin on, first cut the ply to length. Make sure the spars are running parallel to each other. You are about to put a lot of pressure on the spars, if one sticks out more than the others, it will be the cog that sticks up, and instead of getting beat down it will pop out(trust me). My strap clamps started out life as cargo straps. The straps are hooked to the frame underneath and I never found a good place on top (good luck). Once the glue dries trim the excess off both ends.
Repeat on other side.
Fillet in the spars and joints with fiberglass. A coat of fiberglass rolled on everything would be cheap insurance against moisture. Side note the second skin buckled as I bent it, should have removed it and installed a new piece.
Here is the only good picture I have of the trimmed tail. For now ignore the tail lite.
I added a wall at the end of the cabin and started framing the counter top. The shelf at the right is for an ice chest. I also added the battery box. The triangular ply (1/2") is for the skin.
Ice chest in place. Yes I am vertically challenged so if the chest sat on the counter I would need a ladder.
I built four spars for the tail, plus two lower half spars. In between the spars I put braces level with counter. Everything above the braces is part of the hatch (do not cut yet). The top of the spar hooks to the hinge piece at the top. 
Since the battery will be in this space I added drains for the lower floor. I used 1/2" pvc fiberglassed into the corners. I used a piece of square tubing to support the next piece. I framed out the lower half of the tail.
