The on/off switch and the intensity control are mounted on the circuit board. The mode switch (normal/harmonic) is a push/pull pot that replaces your volume. In practice, no one really diddles with the intensity pot; it's kind of a set and forget thing that could be replaced by a trim pot that wouldn't require an additional hole. You will need a hole, though, for the on/off switch and you could replace the push/pull pot mode switch with a miniswitch and drill a second small hole for that. Either that, or rethink your volume/tone control options.
Don't forget, you need a place to put a 9v battery, too. And, if you haven't used a sustainer before, it works much better with newer strings. The extra brightness seems to help.