Don't know about doing it on a bass case, but what I've done with really awful musty rugs is: Sprinkle an entire box or two of baking soda over the entire surface, i mean really liberally cover everything with the baking soda, and then leave it alone for a good long while - an entire day if possible. Then dump, scoop, or sweep out the bulk of the baking soda into a garbage bag, let the dust settle, and then vacuum vacuum vacuum to get out the rest of it