I just wrote up a description of a sling I made that can be installed without any modifications to a rifle that has no swivels. It was for a slide action Winchester. I plan to make another for my lever rifle.
The description is in the D.I.Y. Topic section.

Buy one of the muzzleloader style slings that has a butt cuff, sling and then mount a removeable Uncle Mikes to the mag tube-- or if thats not a go, you can thread it through the mag tube/barrel gap and tie it.

I have found that mag tubes are usually too close to the barrel to be able to insert a piece of leather between tube and barrel. I would also be very hesitant to use the mag tube to hold a sling since I doubt that the mag tubes are strong enough to handle a sling without being deformed.

If you go the removable style uncle mikes barrel band for the front swivel, you could get it barrel diameter if its a 1/2 mag. Frankly, I have them on my rifles on the mag tube and have not had one come loose in 45 years use. I also have them on kickers like the 45/70, 50 Alaskan and 50-110 Winchester, again with no problems. You could something of less diameter than leather I think, if you had to.