The shell holders are usually screwed on and can be removed ftom the sadle if not wanted.
There is also a mount that mounts between the stock and receiver made by Burris. I think I've seen variations of this with a picitiny rail as well but could not find it in a quick search.
Another option is to buy a smooth bore barrel with a cantilever mount. They are harder to find these days but are still to be found on the used market. I have one that I modified for choke tubes and it makes a nice turkey barrel.
If you have a vent rib B-Square (I think) used to make a mount you could fasten to the rib
You may find an option that clamps to the barrel but those are hard to keep secure.