A FET that handles that much power requires substantial heat sinking. You could make a hole in the seal for the sink, so the switch is inside and the heatsink i

A FET that handles that much power requires substantial heat sinking. You could make a hole in the seal for the sink, so the switch is inside and the heatsink is outside, but then you have to worry about how this will affect the integrity of the seal. It has to handle high temperature and pressure.