Trammel Points allow you to draw out a really large, accurate circle.

Attach two trammel points to a long 10mm square bar of metal or wood, adjust their distance to the radius of the desired circle, and scribe your circle.

Print two of these, partially drill out the through 3mm hole and insert a sharpened bit of 3mm diameter steel, cut-off M3 screw, or whatever fits. Don't drill through, as you want your pins to only go in so far.

Clean out the 10mm square holes, to make a tight fit on your rod of choice.

Slide the points onto the rod set their distance, and lock them down with the M3 setscrews.