BMW cheats by molding the top cover material around the
stock hole to provide a smooth fit. My fit wasn't as nice.
I used 3M Tremflex Rubber Splicing Tape ( which only sticks to itself) to surround the switch so I could more easily remove
it later if necessary . I used Gardner Bender Liquid Electrical
Tape to fill the gap between the switch and the Carbon Fiber.
Interesting to note that the CF plate is a different thickness
here so the switch is mounted slightly lower than the stock position( which might be a good thing here ).
A rotary Dremel file ( .124 inch ( 3.15 mm)) and a 0.60 ( 2.7 mm) sanding drum were used to smooth out the hole edges. However I don't think there is a way to bevel the
CF plate itself.