I have a technique that works perfectly. First, I take a pipe with a 1 1/4" I.D. I mark where the center of the hole is on the foam and then center the end of the pipe over the mark and press down to compress the foam. I have another pipe with a 1 1/4" OD that has one end filed from the inside out so that there is a very verry sharp edge on it. I place this pipe inside the larger pipe and push down on it while turning. It pops a perfect hole in the foam every time with both open and closed cell foam. I have a smaller 1" pipe set to pop the hole for the flash. You can also try to just heat up a pipe with a butane torch and then burn a hole out of the foam (but I have never tried it).