I would like to do this. Is there a brand of switch that fits in the standard panel slot? Do you remember what kind of keyed switch you used?
The switch was/is not in the panel (I'm assuming you mean the electrical panel). If you have a circuit breaker to run the macerator, all you have to do is install a switch (with a 20 amp capacity) in the positive lead between the electrical panel and the macerator. It can be any place convenient and you can extend the wiring if needed.
I think I bought the switch at West Marine. It was not expensive. You have a choice of momentary (you have to hold the key in the run position) or maintained contact (turn the switch, let go, then turn it off when finished). This is more convenient.