I wouldn't even entertain the idea...
Using it as a decorative insert might be feasible, but that's 'old' fur and if it were to become a using holster, it would soon be reduced to dried skin.
Furs like the one described aren't rare, not by a long shot - most antique clothing stores and many estate sales have them, and most can't be worn because they weren't stored properly, 'and' because females got far larger than they were during the heyday of the fur coat's popularity, so they're on the 'too small' side.
If you do an insert, make sure you can easily replace it, and keep enough of the pelt to do so.
Scouts Out!