by noseoil » Thu Jul 12, 2018 7:02 am
Depending on the actual shape of the box itself, I would also encourage the use of a "skeletonized" wall system as Tony has suggested in his post. The main thing to remember here is that the roof load must be transferred down to the trailer frame, creating a "load path" which extends from the mounting feet of the roof rack, down into the roof joists, down the side wall framing & finally into the trailer frame itself. Ideally, it will then be distributed around the metal trailer frame & absorbed by the structure of the frame. You will have a wind load when towing, then a down-force load when the tent is up & you're sleeping int it. There may also be some lateral loads as well with the tent deployed, but this is a family site so I won't go into that type of loading & the forces involved...
I would start with a simple line drawing showing the rack's feet & its location on the roof, the mounting points (hard points & fasteners on the roof), the frame members to distribute the down-forces & load path, finally the attachment to the trailer frame on the bottom where it will sit.
You could build with 2x6 walls & roof rafters, heavy plywood construction to make it very strong, or make a much lighter structure with framing members (the skeletonized plywood approach), thin skins & some blocking for the hard points where things attach. Most people tend to overbuild in terms of strength & weight (myself included) but often it isn't really necessary & adds nothing to the overall strength.
If you don't use a skeletonized plywood wall (3/4" plywood with 1/8" skins would be plenty strong enough for a 4' wall height if the skins & framing are fastened & glued in place), I would use 1x2 framing for the walls with 1/8" plywood skins (either plywood or softwood for the vertical members). That way you could use a rigid 1 1/2" foam insulation, 2" hardwood roof joists for the attachment points & softwood the same size for the rest of the framing. 2" foam roof insulation would be a good amount of insulation & provide support for the 1/8" plywood skin. A rabbet joint for the roof at the edges would be the best way to attach the roof system to the walls for strength, weight & durability.
You have a lot of pondering to do here. Best way is to draw it up with simple sketches & continue to refine the simplicity, weight & strength until you have a system which looks good & works for your needs.