you could use one of these, I did and it works great
http://www.stewmac.com/shop/Electro..._Switches_and_knobs/Mini_Toggle_Switches.html i used just the on/on one but wired it so it was an on/off
the bottom has 6 prongs , 3 on each side, if you want to use it as a kill switch you only need to use one side. It's easy, just unsolder whereever your output wire goes (not the ground) and solder it to one of the 3 prongs on one side(test the order to get the on and off in the positions you want, i.e. up=on, down=off or vice versa) then solder another wire to the prong right next to it and then solder it to where your output wire originally was, then the last prong you'll want use as your ground so you don't get any annoying buzz or electricuted. sorry if this is confusing, but i just did this mod and thought I'd share, good luck!