peak dock
It's should be a buy-able item, just like the quiver, that you can put on either tactical hip slot to grab ammo like shotgun ammo and revolver ammo. (excluding RPG ammo) This item should have a storage capacity of around 20, once again like the quiver.
If possible, another feature of this could be lowering your weapon of choice om the ammo pouch to automatically reload it if the required ammo is in the pouch.
(Reference Below)
wicked jungle
pallid fox
woven verge
fiery crater