ANY shortened antenna is a compromise, no matter the frequency.
If it were mine, and the bed didn't have a shell on it, I'd go to www.geotool.com
, and purchase an NMO stake pocket mount, and stick an Larsen NMO2/70B on it.
If you don't want to use a stake pocket mount, a Larsen angle bracket will work very well screwed to the bed rail.
Almost with out exception, the import antennas are not rugged enough, they don't have the gain they say they do, and although their NMO mounts are similar, they are not fully compatible with the Motorola ones.