My 05 Monster 620 has threaded holes on the Coffin Reservoir clamps that I believe the OEM Mirrors bolted in to. I found some perfect black plastic circular caps that fit on and cover these holes but I havent found a good adhesive to keep them on. I've tried Loctite Super Glue but that didnt cut it. Any recommendations on a good plastic to metal adhesive that will tolerate the vibrations and temperature fluctuations at the handle bars?

I've had good luck using the blue color silicone you coat gaskets with. On mine the mirrors were in the holes that's actually part of the bar clamp.  I just turned mine upside down.

^^^
+1
flip em

The best solutions are always the simplest. Thanks!! 

JD weld. Epoxy that stick to anything.
My barend side mirror broke so went out and bought gorilla epoxy but it only lasted 2 days.
I use JD weld on everything. Used on my laptop, desk, air mattress etc.
They sell at walmart.