I through up a nice fishing rod holder on the wall to hold all of our whips. Works great.
I have quite a few old ones that I keep around for spares and installed a 4" piece of PVC on the roof with a cap on one end and a male adapter and plug on the other.
I have a 4'' tube caped on one end open in the other strapped to under my lower shelf in my enclosed trailer works great I have like 8 tips in there and still room for plenty more.and thay never slid out the open end.
We had a toy hauler that had a 3” PVC pipe strapped behind the rear axle of the trailer with threaded caps on both sides…. It was perfect and you wouldn’t even notice it was there unless someone pointed it out.
something to consider.... if your whip sits flat on the roof of your car, you have to put something down on the roof before you fold the flag down. or else.............. the whip will rub straight through the powder coating to the metal and you will have a 6 foot long line down the middle of your car ........ ask me how I know........ LOL.