I don't know either. I have used a #11 X-Acto blade, and it works, but my preferred method is a very small, half-round, burr cutter in a Dremel. Quick, easy, and very little chance of the knife going way too far.
I have also used an ordinary drill bit with an wider included angle than the standard 118°; although, a 118° drill will work. 135° is what I have. Put it in a pin vvise and drill by hand.