This band saw circle jig is really easy to build with some scrap wood and a few pieces of off-the-shelf hardware that you may already have laying around. You will likely have to adjust the size to fit your particular band saw, but here are the dimensions I used to help get you started.
The jig is made of two base pieces bevel ripped apart to form a tongue. The base pieces attach to a backboard, holding the tongue between them while still allowing it to slide. The sliding tongue has a nail driven in one end and trimmed to act as a pivot point. To adjust the pivot, move the tongue and lock it in place by tightening the thumb screw in the backboard.
The jig is held to the table with a pair of turnbuckles and blocks. To use the jig, mount it on your band saw with the pivot nail aligned to the front of the blade. Adjust the tongue so the distance from the blade to the centerpoint of the nail matches the radius of your piece. Lock the tongue in place you’re ready to cut.