Insufficient wheel-axel friction

[ManicMechanic]

With increasing time and wear, we are finding that wheels, formerly held on by friction, routinely fall off. While it would be far more robust to secure the wheels by affixing a collar at the end of the axel, 3-inch axels aren't long enough to do the job. We are thinking of cutting our long axels into 3.5 or 4-inch lengths, but were wondering if anyone has a cheaper solution.