I used the same mounts for some window/door take down tools on top of a swat truck. They hold just fine going down the freeway and those pieces weigh more than your shovel.
Always good advice. That said, I will attest that I've had a pair of quickfists on the side of my van in harsh daily sun (driveway faces such that the QFs always face south) and they have stayed pliable and functional over the course of ~8 years thus far.Further checking for any degradation from sun or age would be high on my list.
I do anything I can to avoid mounting anything on the outside of a vehicle.