4 kayak trailer made from the Harbor Freight 4x8 1195lb folding trailer. Kayak frame made from Superstrut & Unistrut. Frame lifts off in a single piece so that the trailer can be reconfigured as needed, or folded away.

Trailer modifications are running a ground wire to each light (don't bother trying to ground through the frame), using split wire loom tubing (about 30 feet) and conduit straps to protect all of the wiring, and using longer bolts on the trailer hinges to stop the hinge bolt from overhanging the floor of the trailer.

Parts list for the frame build:

3x 10 foot Unistrut/Superstrut
1x 2 foot Unistrut/Superstrut
4x https://www.mcmaster.com/33125T479
4x https://www.mcmaster.com/33125T486
4x https://www.unistrutohio.com/unistrut...
32x https://www.homedepot.com/p/Superstru... (comes in bags of 5, so 7 bags)
40x https://www.homedepot.com/p/Everbilt-... (I bought a bigger box)
8x 1/2" flat washers, spring washers and nuts (used to attach the bottom brackets - could be avoided if you flipped the bottom strut to be channel up)
4 pairs J-frame kayak holders

The following cuts were needed:

2x 78" - cross bars
2x 40-1/4" (approx) - top rails
2x 48" - bottom rails
4x 12" - vertical risers

If I were doing it again, I would consider make the bottom rails longer than 48", and attaching the rear U bolts closer to the rear of the trailer. As you can see the rear beam U bolts are on the very end of the frame. The U bolts have an exact 1-5/8" opening, which is the same as the strut channel and the trailer frame. The suspension setup results in parts of the frame being wider than 1-5/8", so this is a limitation as to where the rear U bolts can fit around the frame unless you were to extend the bottom strut channel further back. Don't forget about ball weight though - you need the center of gravity forward of the axle.

Assemble the frame by attaching the brackets to the 12" vertical risers first - this allows you to get the top and bottom bracket heights identical - then make the rectangular sides on a flat surface to keep them square, then attach the cross bars last using the trailer as a guide to get the spacing correct. The trailer is about 48-1/2" wide - measure this and then install the cross bars accordingly to ensure that the frame is a direct fit. It's important the width of the frame exactly matches the trailer so that the U bolts work.