You need to take a piece of 2" PVC and cut teeth in it, drill a hole in the other end to stick a pipe, broom handle, etc to use as a lever. twist from side to side to bore the hole in the foam.
When you drill your hole on the inside, go as low as you can in the "V", approx 1 1/2" in you will hit a boss. Using your hole saw, drill right through that, it is only there to hold the bulkhead in place during construction of the hole. You want your hole as low as possible.