The ideal way to do it is with a "Hot Bow". This is a device that looks like the bow from a Bow and Arrow, but the "String" is a piece of NiChrome wire that heats up and melts throught the foam. You would also need to create templates to guide the wire on for a smooth cut. All in all, it's a somwhat complicated system, but anything else is really going to mess up the foam.

If I were you, I'd just leave it as is.