Can't it just be a piece of ferromagnetic metal in the phone with the actual magnets on the charger? That's how I stick my pixel 2 XL to a magnetic mount in my car, just have to use a case and put the piece of metal in it since the phone is not made of enough ferromagnetic materials.
Would be nice to avoid the case but, after experiencing the pain of no headphone jack, i will not be buying any phone without one so, Pixel 4a it is for me. Though I'll be losing screen resolution, the screen is also smaller so I likely won't notice (but the ability to take high resolution screenshots was nice regardless) and the 2 XL was always just slightly too big anyway. It's a steal at $350