I friend of mine created a custom mount for the GMRS-Pro radio. The mount holds the radio’s belt clip and should provide a good fit. I’ve ridden hundreds of miles with no issues.
The 3D file is here: BTECH GMRS-PRO radio clip mount by bobko69 - Thingiverse
This mount is specifically setup to accommodate a RAM Ball Base:
Amazon.com
It also has space for the USB-C charging cable, so you can ride and charge at the same time.
I’ve mounted it to my bike, as detailed in attached pictures.