Your phrase is a bit poetic, and metaphorical. It certainly isn't something you would hear in normal conversation. A 'husk' is the shell of a seed, such as wheat or rice. It is a hard, protective, close-fitting cover. The writer is using that idea to describe the banjo case. Like many musical instrument cases, it is made out of leather that has been moulded around a former, so that it is the same shape as the instrument.