Unfortunately there has never been a good solution. The whizzer bikes did it best for the application (IMO) by adding a belt sheave to a standard rear cruiser wheel. Hub adapter on a Shimano coaster seems to be the standard for budget builds.
It seems the cheapest way to power a wheel these days with also a disc and freewheel is to go electric, with a hubmotor.
It seems the cheapest way to power a wheel these days with also a disc and freewheel is to go electric, with a hubmotor.