I rigged this up
Bingfu Vehicle CB Radio Mobile Radio Antenna Mirror Mount Bumper Roof Rack Pole Mount Fixed Bracket for Car Truck CB Radio VHF UHF Ham Radio Amateur Radio Mobile Radio Antenna SO239 Female Connector https://www.amazon.com/dp/B07R7RBGXF/ref=cm_sw_r_apan_glt_fabc_2N1QRZ10DJM783Y80DVM?_encoding=UTF8&psc=1
It's a bracket that mounts to the rack, with a thumbscrew to let me tilt the antenna down for garages and other low clearance areas.
It's not this one, but it looks an awful lot like it.