I just contacted Jason again at Tischer BMW. The part number for the new M3 iDrive knob is 65827839520. He says it is "shaped" different than the knob used in the M5 and M6.
I think I'll wait for the correct knob instead of trying to modify the one for the M5/M6.
2011.5 M3 Coupe: Space Gray, 4MY, ZCP, ZCV, ZP2, 494, 5AC, 655, and 752
Mods: Full Venture Shield protection kit, RPi scoops, Euro fog light switch, LUX V4, clear reflectors, OEM black chrome grilles and side gills, black hood vents, IND F10 M5 shift knob and boot