Yes, I install cargo hatch doors, my method is one I have used well but others will weigh in their styles...its all good mostly.
I go to the RV Salvage (Az R.V. Salvage) and buy identical doors often and install the doors on both sides of the trailers often under the bench seating on several brands of fiberglass trailers, buy the insulated doors only, cut your holes for the frames and put a plywood backing reinforcement along the whole wall you cut out, the ply will also have a cut out for the door frame and adhere the reinforcing ply to the fiberglass wall of the trailer after the wood has been first treated.
(I use Thomsons water seal) or marine ply
This strengthens the immediate area, make sure its a snug fit between the bench rails or belly seams and the floor...Here it gets creative and differs greatly trailer to trailer, use common sense, call your older buds together, chew on it with them, then form a plan...Rule of thumb is to return as much reinforcing structure to the area cut out as was taken.
I paid $45.00 dollars for each door, in the size of 12"x40" being insulated doors with locks and frames from salvage.
PM me if you need pics I have on file.
Happy Camping, Safe Trails.
Has anyone ever installed a exterior cargo hatch door on a 13' Burro, Scamp or Casita? Is a special hatch door required due to the thin wall construction or will any cargo hatch door work and what's the best method of installation?