You'd have to modify a National case to get a Singer in it.
A 15-91 should fit in a full size bentwood case, unless the potted motor sticks out in a way that cause problems. I don't think it does though. I've got a couple of full size bentwood cases, but right now they have machines in them so can't check to see if the motor would cause problems. The hinges and base dimensions will not be a problem.