...I finally have a roll hoop that fits!
My little insert for my BSP pipe bender (pictured) worked a treat reducing the crushing of the pipe, alongside increasing the radius of the bend. Took it to very very close to the maximum permissible crush in the MSA rulebook (the MSA is the national governing body for motorsport in the UK, and oddly has stricter cage rules than the FIA).
The MSA state that for the feet of the cage you need a 120 sq cm reinforcing plate of 3mm steel under a footplate (no dimensions given) attached to the tubing of the cage, so I’ve got that all welded in.
Now I’ve got to measure and make up the backstays including their reinforcing plates, as well as a diagonal crossbar:)