I haven't tried this ^ design, and it looks good, but I think his desire for dropouts compromises slightly the overall merit of that part of the trailer.

On my second trailer I used short pieces of aluminium angle to support the *inside* end of the axle, and instead of drop-outs I just drilled holes. He seems to be doing it the other way round. His inside drop-out is cut into the frame(?) and the outside one is cut in a small tab of plywood. Using the yard stick to makes sure everything is in line across the trailer is a good bit of advice though. I assume plywood doesn't split, but that would be my fear for those small dropout tabs. A little bit of metal wouldn't go astray there. Not sure what the rules are for remaining '100% free and recycled materials', (which doesn't seem to be his claim).

I suppose I would've also put some sort of brace or diagonal cross-beam on that hitch arm. Looks as though there'd be a concentration of (leverage) stress at the hitch-trailer link, whenever the trailer goes over a curb etc.

Nevertheless, I certainly take my hat off to the gentleman.