cutting circles
Wow - lots of help on this forum - NOT!
First of all - there are lots of foam cases out there by SKB, Gator, etc, and the mic screens, windscreens, etc are all made out of foam. So to say that the foam will ruin the mic is a stretch!
Now - it's pretty hard to get these factory perfect as the ones from the factory are laser cut. That said, I have had some success taking the foam and compressing it between 2 peices of plywood with the top layer of plywood marked w/ a hole pattern. Then using a hole saw, cutting through the plywood and through the compressed foam until you hit the bottom layer. The downside to this is that the edge of the holes is a bit rough and there are indeed some pieces that flake off for a while - but while not perfect - they turn out pretty good.
I've tried also heating a piece of pipe with a blow torch - and quickly melting through the foam, and couldn't quite get the hang of it without melting in areas that I didn't want to melt - but I suppose you might be able to develop this technique with enough practice.
Finally, after experimenting quite a bit, I decided that by the time you buy the foam, ruin the foam practicing, buy the hole saw, buy the wood, etc., you would have been better off buying a pre-cut insert like this:
diyroadcasesstore.com/microphone-insert-25-mic
That's about it.