[Mike Simard]

If you use 1-3/4 .12 wall. The .12 wall allows you to slip fit over 1-1/2 diameter. That way you can mount 1-1/2 stubs in the floor of the trunk angling forward for the brace to slip over. There's no need to conform to racing wall thickness because the single hoop wouldn't be allowed in any class that specifed wall thickness anyway.
BTW, if you just need a regular Cobra hoop, you might be able to buy one from Factory Five and adapt it. FFR is a good source for odds and ends, good prices and service.