Tinker's got it. Any shape is OK as long as it's big enough to easily centre the ball.

If you lay a strip of cloth over the barrel and seat the ball flush, cut the cloth off and then, from the other end, push the patched ball out, look at the patch. You'll find it's square with rounded corners.

I mostly use precuts today, and generally they are round. The reason I use round patches, is I can cut a LOT in ashort period of time, using either a large ball bearing and the appropriatly sized bearing race, or an arch punch on a piece of UHMW or HDPE. This is the same material used for counter tops in butcher shops and for chain guides in logging/lumber mills.

A pice 12" square by 2 1/2" thick can usually be picked up for about $10.00 - locally here.

I use the 1 1/2" arch-punch for patches for my 20, 16 and 14 bore smoothbores and rifles.

A smoothly radiused crown is of paramount importance in allowing the easy seating of the patched ball inside the barrel. Once it's inside, it is a simple matter to seat it on the powder.