So I basically forgot about this thread until now. Got two letters from the BMV, one thin one for Oktavia (is due for e-check) and a big thick one for my non-emissions vehicles. SMVxxxx... what is SMV? Do I have to lose the RX-7's Bicentennial plate? Was that the plate number for my GTI?
...Oh, it's just my trailer. $39.89 annual registration, for posterity's sake, and its official weight is 157lb.
Where we left the thread, I was waffling on a box. Then I bought the box and lost the receipt. So I can't do an official running tally anymore. I'd like to say I spent $85 at Tractor Supply. Trailer cost a hair under $500 from having an empty driveway to a finished and road legal product.
On to the pictures!

3/4 oak plywood cut and mounted with 3/8 bolts and washers on all sides, tires getting mocked up.

Mounting the wheel posts. This is nothing more than 3/4 pipe and flanges, and some long 5/16" bolts. Plate is some monster 3/16" thick plate I had at work that I had already predrilled to, I believe, 26" centers. Rally tires are no larger than 25.5" so this was ideal. I'm using the plate as a drill guide.

Flanges bolted. The plate serves to support and strengthen the whole mess, steel bolts to steel with the wood merely sandwiched between the two.

And then more stuff happened and it's finished! Fully loaded and ready to go to its inaugural rallycross. And I broke an axle on the drive down, but that is another story.

Gas can fits neatly in the cubbyhole between box and tires and bounces around a little but doesn't go anywhere. Rally wheels serve as convenient cupholders since they wouldn't see any dirt this day.
Top retainers are just more flanges with U-bolts attached. I have subsequently carried six mounted tires on the trailer and could accomodate two more if flipped upside down. And given that it is just 3/4 pipe, I can make the pipes even taller or shorter if I saw fit. Or just remove them entirely if I wanted to carry a different load. Or leave them in place and use them to lash things to.