Barbara, my baritone dulcimette is about 28.5" long and 7" wide at its widest along the lower bout. If your local music store is worth anything, they might be able to order a viola case from one of their suppliers. You could then bring in your Ginger and determine whether it would fit or not. If not, they should be able to return it no problem. It's worth talking to them.
You will have to adjust the inside of the case to adapt it to the dulcimer, but you should be able to see how to do that when you see them both together.
Dusty T., Northern California
As a musician, you have to keep one foot back in the past and one foot forward into the future.
-- Dizzy Gillespie