This is what I've used for just about ever...
Long time ago, my Dad bought all the diameters of brass tubing that were available at the local hobby shop. Whenever we needed to cut out a certain size circle of plastic (or balsa, when we were into rubber-powered flying stuff) we took a knife and started cutting and/or shaving the inside diameter of one end or the outside of the other end until there was a sharp edge. Finishing up with some sand paper to even things out we turned this collection of brass tubing into a simple punch-set.
You could do the same thing with any piece of brass tube, or any other strong-walled soft-metal tubing. Once you've made your punch(es), you can then cut by tapping with a hammer or by rocking and rolling in a circular motion or twisting the tubing like a drillbit. Just remember, brass is soft and the edge will have to shaprened every now and again.
Fade to Black...