If you want a "permanent" sling, you will have to drill the stock for an insert... It is connecting the sling to the barrels that is the issue for a double.
I used to use a clamp that was made for an O/U... Those clamps will take off the bluing, and eventually slide (even though they have a rubber buffer against the barrel).
I would use some of the paracord loops that loop over the barrel but under the rib (if there is any spacing under your rib)...