On a safety ground i would say 38mm CDS with a wall thickness of 3mm as a minimum (BBrules) you can go lighter on the wall thickness if the diameter is increased, (detailed in the BB)
The height of the main hoop has to allow your head to pass under a line to a substancial member infront of the driver (not the fibreglass body/bonnet)
I would go with a main hoop and 2 backstays with a diagonal as a minimum, but the others have suggested the best route, speak to the clubs
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