On FRP panels. I've been looking a construction method that should be mostly able to be done at home with common tools plus a vacuum bagging setup. Boxy with angle iron corners. The curved angles would likely need to be made by somebody else, and the exterior frame welding is also an outside job.
There is an overall angle iron frame on the outside made from aluminum, then the panels are glued to it. After the panels are glued in place, then the corners are filed with expanding foam, and angle irons are fitted to the insides and glued in place. That ties the inside skins together. The back on this hard sided pop up off road vehicle is built using this technique:
http://forum.ih8mud.com/expedition-builds/245430-travelling-cruisers.html There are some shots of its construction starting around post 61. Here is a part of a post I made in that thread showing a cross section of a corner for a hard sided pop-up.
Effectively the frame of his is rounded corner AL angle irons welded at the corners with panels glued into them to make the sides. The panels are pre-made composite panels.
This is a quick detail for the box corner I designed showing both the lifting top and the base in a 3D cross section. Lifting top has the curved corner for the outside and a angle iron for the inside. The base has both inside and out as angle irons. The red pieces are delrin plastic glides, and likely need to be thinner to prevent binding. The panels are 1" thick AL/foam/AL composite panels. Their skins will be on the order of .032" thick. The outside curved corner can be formed from a 1/8" thick sheet AL plate. No need for a custom extrusion of that shape. The panels will be glued to the outside corner pieces first. Then the inside corner angle irons will be glued in place to finish off the corner. The inside angle irons are not welded to the frame. They are only glued in place.
I've been looking into making fittings for the corners of the outer frame so that I can glue and rivet the angles together at the corners instead of welding them. I don't have the needed welding equipment. Also the above cross section is for an off road vehicle. It is using 0.125" thick angles in the corners. I figure 0.080" or 0.100" would do fine for a TTT.
The angles can be bent onto long arcing curves, but that obviously takes some metal working. It can be done at home with the right hammers, and anvil, a leather sandbag, and a torch to anneal the metal. For annealing get some means to read the temperature like an IR thermometer, or those temperature indicating wax pencils. You need to know when the aluminum is up to temperature to get it properly annealed without overheating it and destroying the alloy's properties.
Angle iron corners with a 1" to 3" radius curve instead of the tight 90 degree angle can be made on a slip roll, but you need a long enough one and that machine costs $$$. It would likely require having that done by a fabrication house, but it can be done. There is also the possibility of doing it yourself with multiple uses of a press and some forms, but it would take some trial and error to get it right.
The other technique is to go with fiberglassed corners. A few layers of fiberglass would be laid up over the corners and edges. Much messier. and lots more fumes.