Soaking in water will make it easier to cut through--just about any kind of small handsaw should work, though it might be a bit of a pain. Alternately, you could snap pieces off with a pair of pliers, or tape around where you want the edge of the hole, put something against it on the inside, and then hit the bare spot with a hammer. Or just smash stuff on it and see what happens.
None of those are perfect, but any should produce some sort of hole.