Plugging Stock Mirror Screw Holes on Lever Mount

Anyone have a solution for plugging the stock mirror mount screw holes? Where to get a plug or reverse threaded bolt. I believe they are reverse threaded. I replaced my stock mirrors and want to still have a finished look.

Order this Yamaha part number:

90338-09012-00

It's made to plug the threaded mirror mounting hole if no mirror is used.
It's black rubbery-plastic.
It can be seen on a Yamaha XT-250G (1980) parts catalogue on the page that shows the control cables.
Lists for $3.63
I've used them.
They work. :)

Remember..... The clutch side is standard thread, the throttle side is reversed.

Sounds like a nice clean solution. Thanks!