That's the same problem I'm having. The nuts that hold the windscreen on vibrate themselves loose very quickly.
I thought about using a spring washer or even an o-ring as you suggested to provide some sort of force against it, hopefully tensioning the nut so that it will not just vibrate loose.
Either that or I'll just have to replace them with nylock nuts and be done with it, and just use a tool to adjust my windscreen. I really didn't want to do that but if it's what I have to do to stop it from undoing itself, so be it...