Coachgeo
Explorer
Looks like you got makings of a roof rack too. Sweet. Now buy a used Travel Trailer and just transfer all the pieces over from it to the box (after you insulate etc.) IMHO consider commercial Spray on foam Insulation after adding Studs you intend to mount cabinetry and, interior walls to, along with windows and laying in the wiring inside conduit etc. Spray foam on inside walls and cieling followed up with a good interrior wall will potentialy give you a bit of a sandwhiched wall construction which should strengthen the whole thng over all as well as giving you good Rvalue.