Keep the kill switch inside the cowl, but try mooving the servo that you use to activate it to the inside of the fuselage and use a plastic pushrod. Another possible solution might be to install a better grounded shield on the sparkplug cable or on the servo cable.

Keep the receiver antenna as far away from the engine as possible.

On FM systems, sometimes if the transmitter is too close to the receiver, you get some servo moovement, try mooving the transmitter about 10 feet away from the airplane.

Hope this helps.