One more thought on the roll bar design.
I am thinking that the rearward supports should be quite low, at the same shoulder level as the safety harness mounting points, and tucking into the rear corners, thus leaving a bit of space for beers and BBQ in the rear.
Still need to create some kind of cover and mounting point for the battery in there too, as the MX5 wiring loom and positioning was in the trunk/boot.
Another crazy thought was to use box section, repeatedly pie cut and shaped to form the bends needed and welding all the cuts, shaping with flap wheel and painting up.
Crazy yes, a load of work, yes…… BUT.