[TonyDuell]

This trick has saved me a lot of time and damaged panels over the years (I used it again today which is what reminded me of it), even if it does involve 'wasting' a slide switch.

When I need to make the holes for a normal type of slide switch, I start by dismantling a switch of that type (I only use a few types of such switches, so I keep the dismantled bits from one project to the next). Normally you just have to bend up the tabs on the back and remove the insulating contact plate and the slider/moving contacts. The part you need is the body.

Now position the empty body on the back of the panel and mark/drill one of the fixing holes. Screw the empty switch body to the panel with that hole, then mark and drill the other fixing hole. Screw the body in place with both screws.

Now drill as large a hole as you dare in the rectangle where the slider will go, and file it out using the body as a template. You will then find that your complete switch fits properly and that the rectangular cutout is neater than you could probably make without the template.