Hi!
One method I use on all my models is to mount the switch inside the fuselage on a 1,5mm plywood plate and fasten a thin 0.75mm piano wire to it that goes out trough the fuselage side where I bend it back (L-shape). I finally solder a small small blob to the piano wire end. This On-Off switch just protrudes 0.5mm outside the fuselage and isn't noticeable that much.Se picture below.

Then last year I saw an On-Off electronic switch that could be mounted inside the fuselage. It has two buttons. I mounted it behind the plastic covering (Oracover) on my Q-500 racer. Touch the plastic surface and it's ON ...touch slightly to the right and the switch goes OFF.
To know where the touch bottoms are I have marked the surface with small plastic film dots.