Years ago we used wheel collars to keep the wheels on the axles. Here is another option. Use #4 flat washers and small silicone fuel tubing. Use this sequence. Put the wheel on the axle, then the washer, then squeeze on the fuel tubing before you cut it. If you cut a small piece and then try to squeeze it on, it will be difficult. Cut the tubing after it is in place on the axle.