I don't have a lathe but I like the way you think.
So I going to put this hole saw over a plastic handi-wipe container and then put it on the drill press.
I'll slide the foam over the the handi-wipe cylinder and then use a razor blade on the drill table to make the cuts square.
It's just wacky enough that it might work.
I haven't cut mine yet but I have some kind of a scraper that takes 4" razor blades. They're available as packs of replacement blades from Lowes et al. I planned to try that.
I'm resurrecting this old thread. The foam rubber "donut" on this M5 I just bought is in sad shape. I remember ordering one using the e30 M3 part number years ago, but can't locate that now on RealOEM. The part # listed in this thread above is not correct.
Does anyone have the correct e30 M3 part number for this thing? If so, please let me know. Otherwise, I'll try to create something out of a pool noodle!!
