I've see a short picatinny rail that mounts to an existing sling stud, it looks to be about 2"-3" long. If you have a stud on your fore-end, that might be one way to go...except...the fore-end might have to be relieved a bit so the rail mounts completely paralell ( mis-spelled) to the bore axis, due to the "belly" on the Marlin fore-end.
Mark