My older brother took a magnet from a speaker (old car speaker, those are pretty strong magnets), covered it with cloth and used that to secure the gun in his truck.
Now-a-days, everything being made with some type of plastic, I am not sure you could use the speaker magnet.
Drilling and mounting you definitely have to be careful. Had a car in the shop once where a guy had mounted some cup holders to the floor of his car. Apparently when he drilled the holes, he only had a long drill bit and went right into the top of the transmission. (Front wheel drive vehicles SHOULD not have this problem. ) Cost him plenty to have it fixed.
I like the idea of using the holster, but I would not want it permanently affixed to the vehicle. Gotta get these cobwebs out of the old noggin and look at a way to mount mine... Velcro on one side of the holster should be enough to get it to attach to the carpet in the car...