Niles,
The cell in my Ausitn Healy Bugeye race car has been in for about 11 years and doesn't seem to be breaking up yet. For an opening, I used a hole saw made for metal to make the hole in the foam for an opening to measure fuel level.
I chucked it up in my drill press, set it on high rpm and just held the foam firmly in my hands and raised it up into the bit. Made a nice 3" hole through it. Blew the small bits out with compressed air and went racing. Works fine.
