Not tying the cage to the rear shock towers seems like a missed opportunity to reinforce them and maybe run a rear coilover setup.
The Classic Car Club of Manhattan Z4MC has a nice cage design:
https://classiccarclubmanhattan.com/...o-part-review/
as does this example:
https://www.panjo.com/buy/2006-bmw-z...ack-car-346814.
Both of these have been around for a while and have build threads as near-top results when you image search for caged Z4M coupes.