After an extended hiatus, here is a bit of an update on the build so far. I've been idle, yes, but not completely idle.
I made the bottom of 7/16" (top) and 1/4" (bottom) plywood, with 1/2" framing and insulation between them.
I coated the bottom with 2 half strength, then 2 full-strength coats of spar urethane, over which I applied 2 coats of Ultra-roof 1000 elastomeric roof coating (this is instead of the roofing tar). The picture doesn't show it well but this stuff was brighter than the sun - I actually had to wear sunglasses when I put it on.
When I went to attach the floor to the trailer, I saw that there would be a tiny gap (1/8" or so) between the frame and the floor due to the fact that the cross members sit a little bit lower than the side rails. After some suggestions from some helpful folks in the tips & techniques forum, I screwed some aluminum spacers down in the middle to provide support.
The sides are 1/8 - 3/4 - 3/16 plywood, with insulation installed in the cutouts on the middle piece.
I'm going to try to stay busy on it to have it ready in time for Fall/Winter 2013...