Loose flash on hotshoe

I've got a Vivitar DF-383 flashgun which works great except that lately it isn't tightening enough to the hotshoe and will slide off, no matter how tight I screw down the plastic wheel. Part of the problem (I think) is that the mount is plastic (i.e. plastic threads) and the wheel is plastic. Therefore when I try to tighten it sufficiently, it feels like it is jumping the threads. Anyone have any tricks or suggestions (other than gaffers/electrical tape) to help get the flash to be nice and tight? Thanks.

If replacing parts isn't an option, you could build up the sides on top of the flash's foot with something.

I tried a slim rubber washer cut out to fit around the stem, but it was still too thick. I'm trying to think of materials that are nice and thin that I can trim to fit in that area. Thoughts?