Glass mount on VHF is going to be about 38 - 42 inches long. When I've seen cars with these on them, they always look like crap (personal opinion). With the metal film in a lot of tinted glass, mounting them can be a real issue, too.
I've installed at lot of permanent NMO mounts on leased vehicles, never once had an issue when I turned them in. The dealers that accept them back very often send them out to clearing houses, and they really don't care what they look like. An NMO mount with a cap on it, or a plug installed in the vacated hole, is going to draw a lot less attention than you'd think.
Fender mounts work well, but can cause funky radiation patterns due to the offset ground plane. They'll work for what you need. Vehicle specific mounts (as mentioned above) can make things a lot easier.
Mag mounts just look amateurish, and often the coaxial cable gets damaged where it gets run through doors, windows, etc. Magnets belong on refrigerators, not antennas.
The Chief here at work has a Ford Escape with the AM/FM radio used as the VHF antenna. Not ideal, but looks a lot better than some hack job. Combiners are available to allow use of the AM/FM as well as the VHF.
Personally, I'd say do a professionally installed NMO permanent mount, it'll work better, look better, and save a lot of headaches. When the lease is up, either leave the mount there with a rain cap on it, or pull it out and put a plug in. You'll find the guys taking the lease back won't bother to climb up and look on the roof.