The floor that came with the trailer isn't quite smooth and event. I bit with the angle grinder sanded down some of the bumps, but for a nice and comfy floor I put down a piece of plywood. Seeing how the floor is already sturdy, that's some of the thinnest plywood I could get.

Not easy to see in the photo - along the rail behind the wheelwells is a welded on channel through which the wires for the lights run. What i should have done is cut those off, and run the wires under the trailer through new conduit. They poke in a little, just enough that the side boards needed a cut to fit snug against the frame. Unfortunately the cuts in the board adds a bit more flex. However with the sides bolted o the frame, and horizontal braces to the runners, it feels very firm.

That tan line along the sides you see? Caulk in the gap between the sidewalls and the wiring channel.