Don't know if it would qualify as a switch, but why not use an automotive plug-in fuse holder. They're light and compact, making them easy to mount on the side of the plane. Plug the fuse in just before you launch. It would also serve as a protection against a short circuit. The fuses are available for at least 30 amp, maybe more.

The other thing that comes to mind is a phono jack. They make some of them so the contacts short out when nothing is plugged in. You'd have a plug with no wires on it that you would remove just before launching. You might be able to find something at a shop for the high-powered audio systems people put in their cars.

Hope that helps.