one could fabricate a holder to screw/glue onto the stock from alum or block plastic.
simply drill the holes, as many as you like, and as deep as you like, glue/doubleback tape, screw onto the butt stock where ever meets your needs.
i've even gone so far as to drill holes the right size in a stock to carry extra rounds but it is a metal 'skeleton' stock
you can get as elaborate as you wish with the design.