Lol it's my own design.
I downloaded a copy of the MSA blue book and adopted as many principles from the regs as possible.
A basic cage doesn't require door bars from my understanding.
And besides which, top of the roof is 47 inches off the floor and I tip the scales at 118kg. It's a tight squeeze now, I'd never get in or out with door bars haha need a diet before race day lol.
My main reason for the cage was for added stiffness between the body and chassis. I can still unbolt the body from the chassis, but the cage is now a permanent fixture. It's bolted to the steel sill members, eventually I will glass the a pillars around the cage for a bit more rigidity as they are cracked in places where the rubber trim goes so that will help too.