lmao - I was waiting for someone to say that.

There's a number of ways to do it.
I can attach thin steel to the base or side of the aluminium plate. I've got any number of rare earth super magnets I can fit into the existing screw holes in the original plate. Or attach to the side.
And at the end of the day it doesn't need to be a particularly strong attachment. Just enough to stop the plate being moved by the printers vibration.
I've also got some magnetic sheets for 2d printing on. They might be sufficient with magents in the screw holes. Although if I switch back to abs it'd probably melt lol
Basically there's a number of options. The fact that aluminium is non magnetic - is largely irrevelant :-)

Once i've got a working system I can get some more build plates made and have different surface coatings on them.
At the moment it's attached with kapton tape on the corners. Works really well - but not exactly hot swappable.

Sometimes you just need a nudge in the right direction to come up with a practical system that works.