Interesting! Never seen any beveled factory cabs.

Anyway, I did the baffle notch mod once and after that I just used thick NEMA 3R rated neoprene gasket material because it was readily available to me, though doubling up Altec or similar gaskets should work fine also. The main thing is to not over tighten! It takes very little pressure to hold even a heavy driver in place for a HIFI/HT app, so consider shims/spacers at the screw points if you don't have either a torque wrench or calibrated wrist. If for a prosound app, then I recommend mounting it to the rear of at least a 5/8" thick adapter plate to mount/gasket to the existing baffle.

GM