2003 Mustang Antenna install

[K5SVT]

Good day,
I have an 03 Mustang GT with a spoiler and need ideas on mounting a ATAS-120. The issue I'm having is that the trunk is fiberglass/plastic so that pretty much eliminates the typical K-400 and the like. I currently have Larsen NMO L-brackets for my VHF/UHF and have room for another L-bracket without the spoiler getting in the way. Do you think this mount would support an ATAS-120? This is the best picture I could find of the mount. I know the coax is wrong. I believe Larsen makes UHF coax assembly for 3/4" hole.
Antenex MB8/LBH3400 [MB8/LBH3400] - $22.95 : The Antenna Farm :: , Your Two Way Radio Source!

Thanks,
Patrick