I have used Finish Line Fiber Grip on some parts where the two plastic parts slip all the time such as the joint on the Mirrycle. It helps keep the mirror from rotating on every pothole, but the stuff only last for so long (6-8 month) before I have to loosen the joint and put some more of that stuff in between the joint.

The Internal or External tooth lockwasher should work as suggested. They might leave a little indentation on the plastic which is not really a problem unless you use a split lock washer. If you find the washer still slip, then try some of the fiber grip alone or with the lock washer.