Do you know of some that, um, would be expensive?

About the weight of a flywheel being too much to mount directly to the motor without support, a greater diameter flywheel of the same mass would have a similar MOI as a heavier, smaller one, right? Mass near the centre doesn't help increase the MOI as much, so you could mill it out, to make it spoked (would need to balance it after, most likely!)...

Someone with access to CNC equipment could get something quite nice...