I had suggested a piece of mica for the insulating gasket. Most electrical/electronics supply houses still carry mica sheets. It's an inorganic mineral with very high dielectric properties. Some other things you might put there like plastics can creep or flow out of the way over time negating their usefulness and possibly causing the fitting to become loose.
A thin piece of laminated glass/epoxy (G10 grade) like .020" thick should work, too. That's a stock item in the MSC catalog.

Jimbo