Try your hole punch on this from McMaster Carr:
Part #86075K24 - 6"x6" sheet of 1/6" thick Viton Duro 75 rubber.
Much smoother under the microscope than inner tube, and firmer.
$17.28 Each
For regs that have thicker seats, you can get 2mm Buna-N nitrile rubber:
Part #86495K34 $18.82
The surface on this isn't quite as smooth as Viton, but still good.
Part #86075K24 - 6"x6" sheet of 1/6" thick Viton Duro 75 rubber.
Much smoother under the microscope than inner tube, and firmer.
$17.28 Each
For regs that have thicker seats, you can get 2mm Buna-N nitrile rubber:
Part #86495K34 $18.82
The surface on this isn't quite as smooth as Viton, but still good.