Steel pan in the center of your attic.
Heavy aluminum foil radials along the rafters, making contact with the pan and spanning outwards.
I've seen someone do this and achieve not only an acceptable SWR, but their ability to talk was surprisingly good. Better than expected. Obviously, each installation environment is different. If you have a metal roof above it, that certainly changes things.
Attic dipoles can also work great. I knew someone who had a large house and had one running north and south and one running east and west, and they used an antenna switcher on them. That was their concept of overcoming the directionality that can occur with dipoles.
I put a speaker wire dipole in a buddy's living room and he had decent results with local talk.
But getting back to the magnetic mount, you could theoretically take a big round piece of steel and drill three 3/8 holes spaced apart in a similar fashion to how the radials are fashioned on a Starduster antenna. Then in those holes you could try to angle mount three 2 or 3 foot fiberglass trucker style antenna masts. That would be a better ground plane than what you have going currently.
Understanding, of course, that everything I've spelled out is sub optimal and a compromise. But, still yet, this kind of stuff is fun to play with and see what you come up with. Theory and science are great guides, but installation environments are either friendly or they aren't. Never be afraid to play around.