That’s a great question, I had assumed they were plastic or glass. When I was sanding them to do a little touch up they looked to be fiberglass. Maybe they have been boogered with in the past and were repaired or something. I’ll take a good look when I get home.
Stop by the local body shop, they have body caulking. Its a putty they use for sealing body plugs and such. I have a tube of it myself. Or butyl tape. A little goes a long ways.