I have a Monoprice M1060C
It has wooden ear cups and 3D printable 'open' mod 'cups' and covers are available.
If I were to (offset from the existing screw holes) drill 3x2mm holes into the wooden cup (and the 3D covers) and mount small magnets in the holes, then mount magnets inside the headphone itself:
I'd have the option to easily swap from open to closed.
Does anyone foresee issues?
Looking at these: https://www.umagnets.com/p/disc-magnet-n50-3mm-x-2mm-nickel-plated/
I've searched and found that magnets shouldn't mess with the speaker itself, but maybe someone here knows better or has tried it.