i'd say you could do it with string. stretch it across the middle of the circle enough to not sag. maybe try just two in various places and see if they are barely touching. trying this in several positions should produce the same results -- barely touching and not holding the other in or out. for example, make like a plus sign and check the possible gap between the string. then try an "x" sign with the string and check again. barely push one string to see how much it takes to get them to not touch.