I have the Cobra HH 45 WX ST (Hand Held w/ built-in ant.)...works great for trails, even w/o exterior antennae...but was short compared to the over the road units, gets me about a 2 mile range. I added a magnetic mount whip, and that boosted the range to more like 5 - 7 miles depending upon conditions. The magnet holds tight, no problems. Just make sure the metal you stick it to is spotless...or you will be sanding that spot as you drive...the whip bends as far as it needs to when you hit branchs, etc...it doesn't jar the ant mount.
I jump in and out of an assortment of rigs...for work, so a perm/drilled mount is impractical unless I buy/install a batch of ant's ahead of time...(Hence the hand held....like a mike with every thing built in...no cords, but lower wattage than the boosted built-ins, only 4 watts)
A tuned perm mount would of course give even better range/clarity, but for me, I'm at the point of diminishing return when I tune the ant length on each mag mount point, and then mark the circles, so I can reattach to the same spot w/o re-tuning the ant the next time.
The clip ons are good, and if just to avoid a hole, a good way to go too....just seal the contact points in, after getting a good ground, to prevent rust.
As for parking garages...the whip gives as it hits each beam across the ceiling decks, as no damage has been noted (Lots of different garages too....) ...so I leave it on and let it whip the ceiling decking...the sound when the whip whacks those hanging metal signs can be a surprise for other parkers sometimes...but no sign, mag mount/ant or truck damage occurs....kind of fun after a while...

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Good Luck!