You could try something like this:
and use it to control the battery master ground or the power to the starter or I suppose provide a second ground source to both magnetos.
Now I'll throw in my two cents: I think keyed switches provide no security at all. A canopy lock deters hands in the cockpit. I good chain and lock around the prop deters engine start. But you are the designer/builder so go with what makes you feel comfortable. Most twins have a door lock and toggles for magnetos.