Here's the frame I'll be starting with. I have to wait until payday to pick up the flooring and undercoat, but I'm starting even more basic than that, with some frame redesign:

I'm going to take out the lengthwise piece of angle iron that's between the front and back crossmembers, and replace it with 2 pieces of channel iron, one in front of the axle, and one behind. Then I'll do the same with the piece on the rear of the trailer, because angle iron isn't as structurally rigid as channel iron. Next I'm going to cut the length member in pieces and weld pieces of it in front of and behind the wheels, so that I have a place to mount storage boxes and a deck to.
Course, I might just trim off the sides, and make it 4x10 only, since those pieces of 1/8" angle iron sticking out on the sides may not be strong enough to hold me or the storage boxes I'm planning. Additional crossmembers are still going in. Suggestions?