I just installed an antenna mount on my 2011 JKU Sahara last week. I wanted to go center mount on the third brake light mount, but couldn't figure out how to make it clear the hardtop wiper so I came up with this alternative for $3.
I started with a 12" reinforcing plate from ACE hardware $3. That was the only cost. The plate comes with predrilled holes. I cut the plate short with just two holes. The first hole was used to bolt the plate to my spare tire carrier. The second hole I drilled out 5/8" for the antenna cable (UHF) base. I ground the edges smooth before painting it. I bolted it to the spare tire mount using stainless fasteners, then attached to antenna mount, and finally the antenna.