If you guys ever need a butt stock adapter and barrel adapter to add a sling to your double barrel, over and under etc give this place a try http://fireforceusa.com for the stock adapter look under 2 point sling and the shotgun barrel adapter you will have to call and talk to Brandy or Nick they can fix you up and reasonable prices
Anobody, on this forum can build you a stock saddle and sling lop for your single, double or over under out of paracord that's one of the best and about any mix of color....check him out ....
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)...