Hi
I'm interested in this issue as well, if I fit a cage to my car my state laws will only allow my car to be a 2 seater, I want to keep this car practical and as useful as I can.
I took some ideas from the Beetle cabriolet.
Steve
PS The car with the nice paint is not mine, I just put them in to show what the finished product looks like.
This my car, I've run brace between the B pillars.