Went looking for 3 1/8" instrument cut out templates today and couldn't find any with the little extra notch so made my own. These fit my United altimeter and Kollsman g-meter. It took a few tries to get it just right but print it and cut one out of paper to try before you cut anything of value if you decide to use them!
Also attached is a basic round instrument cut out diagram with all of the dimensions listed.
Also attached is a basic round instrument cut out diagram with all of the dimensions listed.