For the purpose of transforming a Nintendo DS Lite into a Game Boy macro, I had to place a small speaker inside the case in a place there was no holes to let the sound come through.
I quickly designed a small, not perfect, guide to easily drill aligned holes in place.

Works fine, you can see the inside the outside, and the guide before drilling.

Keep the guide in place with some tape !

Diameter of the largest circle is 30mm.
If not made for your size, it may give you an idea of designing it ;)