All bolt-together, from 14 gauge 1-1/2" steel angle The orange in the screenshot is 1x2 boards used to reinforce the 1/2" plywood trailer deck around the opening.

All the steel angle will be bolted together with 3/8" bolts, and the assembly will be attached using carriage bolts through the trailer deck and 1x2 boards.
Might not be pretty, but should be plenty sturdy, even for my driving.
And I can always come back and box it in with some thin plywood.