I will be experimenting by building my version similar to the top drawing there. I do not perceive an issue with the hitch located behind the wheel centre: Bob Yaks, and most other single wheel trailers pivot from beyond the circumference of the rear wheel, although the centre of gravity is usual a lot lower.

I can see a potential problem with lateral sway stability by mounting off the rear most point of the rack. But careful design, accurate manufacturing, adequate bracing and correct mounting should resolve this issue.

When I start to build my new rack, I will probably use 16 mm O/D X 1.6 mm (5/8 X 16 gauge) 316 stainless steel tubing Tig welded throughout, utilising BZP M6 grade 8.8 (min) or A4 marine grade stainless steel bolts/set screws and nylock nuts for mounting.