On Feb 19, 8:41*am, aemeijers wrote:

or sandwich the rubber between thin plywood sheets and use a drill to
just make a circle of small holes to define the diameter.

If the rubber is clamped between two plywood sheets any hole saw would
work, you could use a knife to cut around the inside of the plywood
template (miss-cuts will only be on the waste side of the hole
perimeter, unlike using a can), and any edge cleanup could be done
with a drum sander bit in a drill or a rotary tool while the rubber is
still sandwiched between the plywood.

R