Warne makes two steel rails that will fit the 12ga contour, but silver soldering would be required unless you can find a gunsmith that will drill and tap it. You could probably get away with one 8-40 screw as a recoil stop and epoxy an aluminum rail on, Superweld is some pretty good stuff, specs are in the FAQs in the barrel fitting info, it has a shear strength of 2700psi.
Tim