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which one kinda depends how you want it wired up.
if it's setup with the quick release box mechanism (with the 3-pin socket on the front so you just slide the box back and out) you could look at a&k's mosfet that they use in the likes of the pkm.
otherwise you can just manually wire in a generic mosfet. for a support gun you don't need anything fancy seeing as it's only for contact protection.