OK here's one...
How about a bracket across the top of the tire carrier, make it out of 1.5 or 2 inch angle, alum OK. Make it as wide as you need and run a support, make out of smaller angle, at an angle to the center of the carrier. That would give you a mounting location anywhere along the flat of the angle stock.
I just don't like mounting the antennas too low.
The distance between the antennas is not critical because of the freqs. but I kept mine as far apart as possible so they didn't couple.
My latest project will have the mount for the tri-band VHF, with fold over mount, mounted centered on the windshield frame, with the mount towards the rear, over the top. The CB antenna, a Wilson Flex-i-whip, will be centered on the tire carrier, both antennas as high as possible and flexible.
Anyway, I an going off the subject...
The mount in the pic is sweet and should work out great.