This is what my trailer design looks like at this point. It has an outer frame of 2x3 beams with 1x3 pieces running across the middle. There is a 1/4" skin on the bottom and a 1/2" floor on the top.
The axle mounting areas are reinforced with 2" x 1 1/2" steel angle pieces. The tongue is two 6' sections of 3" x 3.5# steel C channel. The coupler will be a standard A-frame coupler. The frame also has a piece of the same steel angle reinforcing the front of the frame where the tongue crosses over.
The angle would be bolted to the frame and the C channel would be welded to the angle.
I would cover the underside and sides with PMF to reduce the likelihood of water getting into the frame as I don't want it to rot. The topside would be covered with the actual trailer build which would also be sealed with PMF. I'm probably going to use a leaf-spring suspension system as torsion axles are pretty expensive.
Do you guys see any flaws or problems with this design? I'm not planning on going off-roading with this as I'm towing it with a Honda Civic.