To keep costs down, I'm thinking I'll start with a bolted tube frame design and fold some thin aluminum sheeting around it, and I'll weld it together at a later date. Lay down some plywood on the bottom, bolt the axles to the two beams going across in the middle. Something like 1 inch tubular? Might have to beef it up on the bottom, but given my low weight requirements it might be alright. 400lbs with most of it on the middle should be safe if I use some beefy bolts.
Have some supported sheet aluminum on top with a hinge, probably two sheets so it opens like a box, and can be left open if needed.
Later on, I'll get fancy with the aero-fenders and make them into usable space.