Go to a home improvement store and find O rings that will fit inside the tires. Big stores like Home Depot and Lowes have huge plumbing aisles... you'll probably be able to find either O rings or rubber washers sized to do the trick. If you need to "shim" the kit wheels a bit for the O rings to fit concentrically, wrap a narrow strip of masking tape around the kit wheel to build up the circumference as needed, narrow enough to leave the wheel rims exposed, so the kit tires will seat properly on the wheels.