the outside of my trailer will be covered with 1" rigid foam (pink board) and then glassed. I"m doing it this way so I can sculpt the fenders and have rounded edges and corners (to mock the profile of my PT cruiser). It is a very expensive way to finish a trailer. my build is 50" tall 74" wide ( outside of wheelwells/fenders) and 120" long. You have to use epoxy as polyester will eat the foam. So the cost for just the fiberglass,(2 glass layers and 1 unidirectional layer) , + 5 gals of epoxy come to about $1300.00 plus another 200.00 for filler, $150.00 for urethane primer and $300.00 for color coats and clear to finish it. If the cost isn't enough to discourage you, try fitting a hatch between side walls covered with an inch of foam board, then cover the hatch with foam board, sand to shape and figure a hinge arrangement that will work with it all

Oh yea I forgot to mention the 3 gal. of epoxy ($300.00) to glue the whole thing together and the challenge of fiberglassing 220 sq ft of trailer with 3 layers of glass and most of it vertical and trying to fall off and the endless hours of sanding foam, fiberglass, fillers and paints.

If you still want to try it see my album for the parts I have so far (the foam will be epoxied over the foam on the walls) I will post pics of the whole process as I build it.