I think no one has answered your question directly because the word HF + mag mount = massive red flags to everyone reading and thats why its highly recommended not to mag mount. I do answer your question below but not as you expect.
I would not do a mag mount. Get an NMO mount or install a proper drilled mount. Get a professional to do it for you if you cant. If you are going to keep your car for years and years, the resale value will already be in the dumpster and drilling a hole does nothing to the resale value. I have two vehicles with NMO mounts and its been years and years and the mounts are fine, no rust, no warping, etc.... I run Scorpion Antenna, Tarheel II, and Larsen VHF/UHF whips. NMO, and other custom mounts. With the NMO I can take off the antenna with a twist and put a tiny little dust cap on it and no one can see it and if they do it looks like a modern satellite radio micro antenna with the dust cap on.
To answer your question the only antenna I can think of that is going to be a monoband other than a hamstick is a hamstick. I am sorry. antennas are all monoband. Do not use a tuner mobile they are useless. They are basically useless on big antennas as well unless your running balanced line loops and other non dipole based antennas. Remember that a tuner is a pretend good antenna. they just make a non resonant antenna look resonant to a radio but the antenna is still non resonant and grossly represents a dummy load outside of the tuner. In a car you want resonance to occur naturally in harmony with the mass under the antenna being the whole vehicle and the vehicle its-self is a massive capacitively coupled ground plane to the Earth. Mag mounts sure they work but think of your electrical connection. Your ground plane is as big as the magnets and the entire vehicle is basically ignored. There is only 1 or 2 correct ways to do mobile HF and then there is everything else which is just bad and wrong. Good luck.
What I propose as an actual solution is as follows:
Buy a drill installed 3/8-24 mount, Breedlove makes great mounts. Have it drilled and installed. Or install an NMO heavy duty mount from Breedlove or anyone else that reinforces the sheet metal from beneath. They make NMO to 3/8-24 adapters. Buy a Little Tarheel II and use that. Get a good screwdrive controller either manual or digital, MFJ, TurboTuner, West Mountain, etc... and set everything up. Bond everything in the car to its-self, i.e. door to frame, hood to frame, exhaust to frame, bumper bars to frame, etc... via 1" wide copper flexible ground strap.
Then be AMAZED at what you can work and who can hear you and you can change bands from 3.5 to 50mhz while doing 70mph down the highway never getting out of your vehicle.
or .... you can run a magmount and keep posting questions on how to get things to work better when there is no other way to make them work better besides a properly drilled mount. And even if you drill a mount for your hamstick that antenna will not truly shine until you strap bond everything in your vehicle.
Mobile HF should be completely abandoned unless you are willing to compromise heavily and modify that vehicle. Otherwise youre going to make a contact or three sure. Even coax can radiate its common model so so like an antenna. But I will be making a hundred 5/9's while your still hunting for your 3rd contact. It makes that big of a difference. And it took a long time to type all this so I do care very much for your success.