To finish the floor of the trailer I first had to add some insulation to the back two sections of the trailer. Instead of wedging the insulation at the bottom against the FRP, I rested it against the exposed edges of the metal trailer frame so that it could be glued to the underside of the plywood floor.
We decided that 1/2" thick birch veneer plywood for the floor because it was sturdy and the nicest looking wood they had at Lowes. The first step was to mark out the locations of the recesses onto the plywood floor and to cut them out. I made sure to save these pieces so that they can be used for the recess doors. I proceeded to glue down the plywood onto the wood frame to give it a little extra strength overall.
A snug fit but a good one! I installed a screw every foot along each side of the framing members, making sure to counter-sink each screw so that it was flush. Once this was done, the whole base of the trailer became much more rigid and gave us a sturdy foundation for the rest of the trailer.
When I cut out the doors, I made them slightly larger than the inside of the inside of the recesses. This resulted in a lip around the whole perimeter to support the doors when they are closed.
Before installing the doors, I gave everything a good sanding and made sure to smooth out all of the edges. We purchased some awesome recessed pulls from a local marine supply store and some simple hinges from Lowes. The recessed pulls needed to be embedded into the 1/2" plywood in order to lay flat. I was able to do this by utilizing a milling bit on my dremel. I think everything turned out rather nice!
Next step, side walls and framing. It's finally starting to come together!