This might be expensive for one project but if you happen to have a Babylock or Brother machine, they have a "circular" attachment that sews perfect circles and the sizes are adjustable.
You would sew the circle, then cut just beyond the stitches. Apply as normal.
I bought it cause I thought it was cool. Been playing around with different ideas. Kinda fun tool.
Other than that, I just find something in my kitchen approx. the size I'm looking for and use that.