After replacing these twice on ours I finally looked into the end of the bar and determined that it has a threaded insert and I could just use a bolt which would be much stronger, durable, and would last forever.
I forget what size bolt these are. They are ones I happened to have around but I suspect its a metric size. Its close to 3/8" and is fine threaded and I used a couple of washers on the inside between the bracket and bar to make up for the space lost from the stupid plastic piece. Been like this for a few years and works perfect. Since I used longer bolts I also used a couple of extra nuts to lock them in place against the brackets but you could just as well use some shorter bolts. I just used some I happened to have already...
I cannot remember for sure if I had to drill the holes in the brackets larger or not. I vaguely remember the stupid weak plastic pieces were attached to the brackets with a little hex head bolt or something....