If you are just looking for a way to keep rounded on the scabbard so you can carry them at the range hands free, then there are MOLLE panels that will fit. If you google, MOLLE shotgun shell carrier, you get lots of results. Amazon had a 25 round carrier that looked reasonable for that type if purpose. Get a few and put them on the scabbard. If you don't mind having the shells loose, you could just get a dump pouch.
Ammo cans work great for keeping ammo together and transporting it at the range. One can will hold 100 rounds.
For bird hunting, I don't carry more than 25 rounds in the field, with more in the truck. I use an old SAW belt pouch that just happens to fit one 25 round box of 12 ga. shells about perfect.