Large plastic jar lids can be used for wheels by hot gluing them to the servo horns.
Why glue them?
Position the servo horn (the round one is best) in the center of the lid, mark the holes (2 to 4), drill or punch them out, then use a few small screws (or small nuts/bolts) to hold them in place. Much more secure and easier maintenance (plus you can re-use the servo and horn later).