The Frame on my trials car(s) starts with a Type 3 VW sub-frame/torsion bar housing. The roll bar is regulation 12g 1 3/4" mild steel tube & the rest is 16g 5/8" mild steel tube. I use square section tube because it is easier to match up to the next bit without having to profile it. Everything is bronzed together so it looks neater, and if any cracks occur, it is easier to repair on site. Square tube is also just a bit easier to straighten.

On later cars I've used 3/4" tube as the 5/8" stuff probably bends just a bit too easily (but only when I lean on trees!).

The roll bar fits neatly into the brackets already on the sub-frame. A couple of gussets between the space frame the housing help a bit.