Airbrushing insignia

I have one of those cheap acrylic circle templates used for drawing circles.  I put the tape down on a clean piece of plastic, then use my scalpel to trace out the circle.  I bought a swivel knife to do that, but find that an 11 X-acto or scalpel blade works just as well, so don't use the swivel knife much now.

Gasket punches come in a variety of sizes and come in handy for just this sort of thing. I purchased a cheap set some years ago at Harbor Freight. You can also make your own circle punches from brass tubing. Find an appropriate size and sharpen the edges of the circumference with a Dremel tool or similar. A Waldron or Micro-Mark punch set can also be used; stick the masking tape to some 0.005" thick styrene sheet and punch away. Save the styrene disks after removing the tape; they come in handy for a lot of things, including filling ejector pin marks. Hole punches for leather craft can also be used; they also come in different sizes, and I have a set of eight that I found at a fabric store.