Are there strong but insulative screws?
To insulate transistors from a mounting plate, I typically use metal screws with an insulating pad as I describe in my answer here: how to electrically isolate a PCB from a heat sink.

But if you really want to use non-conductive screws, have a look at fiberglass reinforced plastic ones such as Isoplast. I haven't used them personally and they don't make any specific claims as to shear strength, but they look a lot stronger than nylon.