We're using something like
these fiberglass reflector rods cut down a bit. They go into some aluminum clamps at the bottom that are all pivoted on the same screw. The outside poles have surgical tubing to keep everything centered.
I think it's important to go with something pivoted front/back or that can't bend sideways like those lexan strips, other wise you risk them bending sideways out of the 15". Also having 2 colors front/back or side to side so the driver will know the orientation of the robot.