Calton will make one for you if you supply the dimensions...or at least they used to.

Generally a slightly bigger case tricked out with extra interior padding will do the trick, and the advice above about covering whatever foam with fabric is good. Velcro will hold the extra foam pieces in place but still allow them to be removed if the case is doing double duty.

In my work as a festival instrument security guy I've seen some good retrofits, even an odd shaped mandolin sharing a guitar case with whistles and flutes, all neatly padded and secure. I myself once had a subdivided guitar case for carrying accessories.

Peace, Mooh.