I have received the first 8 trailers from the builder and am open to offers. Email me if you are interested (and are willing to pay for shipping or live in Sydney). I only helped build these, and did the design. Mass production by a helpful friend with a MIG welder.
These are based on my original shopping trailer that I've made at least 10 of now. I've made a rough plan and cutting list for the trailer. The exaggerated overlaps at the corners are to show how I put it together.
This leaves you about 350mm out of a 6.5m length of 20mm square tube, which is just enough to make the left side that runs forward to the bike and perhaps one 50mm bit. So I usually end up using scrap bits instead of the 4x50mm bits, and the 500mm-ish sticking forward of the base is enough. If you're towing with a 20" wheel you can shorten that and get everything out of one length.
I helped a friend build 12 of these recently so I can use them to encourage people to use their bikes for practical stuff. Not a profit-making venture so much as an activist one.
Cost here was ~$200 each, compared to the cheap Taiwanese ones retailing at about $250. But those are very flimsy and rarely last more than about 20 shipping expeditions. My ones... wear out eventually. Phuong's one seems to have been used weekly for about 6 months now and appears undamaged.
12 trailers took two of use about 7 hours to build, then they were powder coated. The real advance over the Moz "do it all by hand" was getting dropouts and hitch parts laser cut. Plus we used a MIG welder which made it so much faster (nice thick wall mild steel... compared to 0.9mm 4130 anyway). I will try to weigh one, but I think they're about 8kg (20lbs) with a capacity of at least 50kg (the cheap wheels are the failure point here). I've put more than 100kg on one (me plus stuff) and it was just very hard to ride the bike.
The process we used was pretty straight-forward - I cut the first four lengths while the welder dude set up, then he started taking the sharp bits off with the linisher. Once we had one complete set I helped him set it up on a welding table and we used magnets to make a rough jig. The flat part went together first, then once he started welding I went back to cutting and linishing. Grinding the sharp bits off makes the welding go better. By the time I'd cut 12 trailers worth he had the flat bits pretty much done.
Then I switched to threading up hitches - first set them to weld the round end onto 4 links of chain, then getting those and threading the spring in and using a nail to hold them together for welding. While I was doing that he welded 24 supports with a 50mm vertical at each end (yes, it was boring).
While those were being welded I cut the angled hitch beams and cleaned up the hitches a bit. Then started wire brushing off the joints ready for powder coating as the welding was completed.
Despite being liberally supplied with whirling blades of death and glowing metal the only injury for the day was acquired from a (manual) hacksaw... the inevitable "bang hand into vice and knock a little bit of skin off".