I made similar jigs, just that I just put a screw in one of the unused holes to hold the sliding part stable.
When I was making larger circles, I made a clamp-on extension table with one dropping leg. Just clamped my one-legged table to the band saw table.
before walmart and everyone else started carrying circular pieces, they were all the rage for a while in the early 1980's. Decorators were putting the circle on top of anything they could find and just draped a large cloth over the top which hung down to the floor. Some cloths were circular and some were square.
I do like the adjustment though. Nice touch
Tony B Retired woodworker, among other things.
"Strive for excellence and settle for completion" Tony B