I'm not sure how any of these caps mount to the body. But if you get one you can "gut" and use as a shell or cover maybe you will need to have an adaptor ring made. Mount the ring to the body then mount the cap to the ring? This adaptor ring would also serve as the trim ring.
Just a thought.:D

anyone know if this is a custom cap or one readily available?

That car was built by Killer Customs. Look up "killer 69" (Blake from Speedtech) and ask if he knows how to get one.