Brown kraft paper, nothing fancy. I had 4 different boards to paint (all different colors) so I just stacked 6 layers (to have a couple spares just in case) of paper and cut them all at once. It’s around 2400 circles per sheet, and took just over 20 minutes to cut.
I did hit up one side with some Scotchgard to keep the paint from soaking through, seems to have worked out.