I've seen a lot of people using steel pipe from HD. It comes in 10-ft lengths and is threaded at both ends. In the same aisle they have flanges with 4 bolt holes that you use to secure it to the trailer deck, then screw the pipe into the flange. It's not a super cheap solution though.

 

You want two of these bolted to the plywood deck. Cover the steel pipe with PVC. There''s about 1/4" clearance around the PVC and center of 6ULs. Drill a hole through the pipes, slide a PVC coupler over and latch it all down with a pin or if needed, lock.

 

I would transport my 15x8 ATS DTC w 225/50/15 V710 standing up three across in the trunk, lid open like a pacman mouth and one in the pax seat. Worked just fine no trailer needed.