drawing arm attachments, post or flat. Score lightly initially like you were scribing door lines then gradually go deeper. Allow for the thickness of your blade cut to determine where you want to cut to end up with your desired edge. You might practice on a piece of your scraps to get the hang of it before cutting on your actual parts.

Google searches for compass cutters and drafting sets will turn up a wide range of prices from under 10 bucks to very high dollars. Maybe someone here can guide you based on their experiences with circle cutting plastic. Have fun!