Custom mount ignition switch

[rujr]

I want to custom mount a mercury outboard ignition in my fiberglass dash. This sounds easy but is difficult because the ignition body, which mounts through the fiberglass, is cylindrical with a flat side, designed to interface with a machined cutout to prevent the ignition from spinning during normal key movement (twisting). Are there any tricks to make a hole or should I use some sort of templated backing plate/face plate to mount this correctly? In the past I have had an 1/8" aircraft aluminum plate water jetted to accept the ignition switch. I jetted 4 screw holes in the corners of the plate, and that made for a solid and good looking face plate for the ignition to mount through. My contacts for the CAD water jetting process are not available at this time. Thanks for the suggestions.