Cool.
I also wanted a centered antenna, for cheap too.
Here's what I did:
https://www.jk-forum.com/forums/jk-cb-ham-radios-128/3rd-brake-light-antenna-install-96042/
Though you might be able to use this idea for grounding purposes. I've removed the tire carrier and antenna mount last week to check for corrosion and none is present. No water has worked it's way in, anywhere.
PM me if you got questions on anything.