A trick I picked up on, a few years back was to take
the face from a junk instrument (57 & 80mm) and use
it as a template. I use them to mark the location of
all instruments and after cutting the holes with circular
saw in the drill press, I use my templates as a guide
when drilling the instrument mounting holes.

BTW, most instrument panels are quite weak without
the instruments, but regain their strength through
the installed instruments. In most crashes, the sides
of the cockpit bulge out, so consider the strength
the instrument panel will provide to resist this bulging
out movement.
JJ