[Goober]

As most gauges have a bezel, measure the actual gauge diameter and make a template. Use a sharp razor (exacto) knife and cut the vinyl or leather. Then mask off the opening and use a jig saw, hand saw, keyhole saw, rotozip or what ever and make the opening. Remove the tape, (I like heavy masking tape) and Bobs your uncle. All done. The bezel will hide small mistakes.
I have done a lot of custom gauge installs and its worked everytime. Do NOT use a hole saw!!!
Get a board and material and practice if your not sure.
Hope this helps

Mark