In case anyone needs it I designed a slimline case for the HAS extender, I had a need for it to be slimmer to fit into a case that I was using for my Lethal Enforcers board.
21.4mm, could be less if you use slimmer screws on top as well.
I had to design that printed plexi for LE, I think you may be right, everyone seemed to put turtles or simpsons in these cases. I also have a SEGA System 16B in one, I didn’t listen when it said it was a Konami case..