I don't think there is a source for an adaptor of the proper dimensions. I made "stops" out of short sections cut from a large cotter pin, glued to the front of the mag. The snap-on extensions from McCormick 10-round mags might be a place to start; there's an alignment tab that would have to be removed, or a corresponding slot cut in the mag tube. You'd then have to cut it to the proper length (assuming it is a bit too long to begin with).