First off, thanks for the suggestions Jdw and junkboy. 2nd, I'm finally happy to report some progress.
First, I clamped some 2x2's down the side rails, inset a half inch for the half inch sides.
Then, I screwed them down with these screws, 4 per side.
Then, I used my fancy new Kreg Jig to make pocket joints and insert the cross members of the frame.
Here it is, all framed up. If you look close, you can see the rips of 5mm plywood I put over the crossmembers of the trailer to level them up to the side rails.
Then I laid my sheet of 4x9 FRP on top of the frame, and cut it down to size using a rotary saw and a rip guide.
Next, I did the same with a sheet of 3/8" BC ply I'm using for the floor. (saving a little weight over the 1/2" called for on the weekender)
Over the weekend, I intend to remove the wood frame and glue it to the sheet of frp on the floor of the garage, then reattach it to the trailer with the frp on the underside, and attach the plywood floor on top. Why didn't I just put the frp on the trailer before I built the wood frame? Because FRP sags a lot in horizontal applications, and I want to glue the frame to it on the garage floor, keeping it as tight as possible before transfering it to the trailer. Am I overthinking it? Probably, but I'm in no hurry so no big deal.
Anyway, more pics to come. Enjoying everyone else's build threads immensely. I'm hoping to be able to make the Lake Bisteneau gathering this fall.