probs
I have some of the elastic 'cuffs, but unless you anchor them down somehow, they run around on the buttstock and end up in the wrong place, in my experience. Sometimes I can take a sling swivel stud out, and catch a corner of the cuff, and screw it down. The elastic tends to wear out too.
I also have a leather one that laces on, same prob. I brass tacked that onto the butt of the coach gun that wears it, and it looks great (4 rds)
Cannibalizing the sidesaddle is working good for me. The ammo isn't as close to the action and requires a bit more reach, but there's ammo on board.
I worked w/ a guy who used a sidesaddle AND a butt cuff, 12 extra rds on board. (no carbines in those days)