Use anything that cuts cloth.
The patches don't have to be round. Square ones work just as well and the gun will shoot just as accurately as it would with a round patch.
That said, if you bound and determined to have a patch cutter to make round patches, figure the size you need.
For a .45-54 caliber a 1 3/8 diameter will work.
Now, find a hollow hole saw. The kind that's made for drilling holes into wood.
Throw away the centering drill. You don't want a hole thru the center of your patch.
Get out your power grinder and grind off all of the teeth leaving an edge that has almost zero runout. That is, when the saw is rotating in an electric drill the edge where the teeth used to be runs in the same plane.
Now, using your grinder grind the edge so it is sharp. You now have a toothless, sharp hole cutter.
Used in a drill press with a pine board to back up the cloth you can cut thru 6 or 10 layers of material in each cut.
These things can be dangerous and can produce some really bad cuts if it decides to make patches out of your hide so be careful with it. :grin: