Why not make a dust ruffle on a flange to fit 3 sides, add an elastic area at the top and put it at the area on the matress needed to be just under the edge of the quilt. The ruffle does not have to be an addition to the quilt making it more yardage to have to launder.
It can be moved up or down on the side of the mattress to fit the drop of the quilt. It can be made of any fabric to either match or contrast the quilt.