barrels
I dunno, Midway is selling the 18-1/2" barrel for the Rem 870, made by Mossberg, now, at $122 bucks, plus shipping and handling. You'd come in about fifty bucks or more below the cost of a Maverick, that would buy a good bit of buckshot. Besides, maybe the OP does not want another gun? (heresy I know).
Another source of barrels is ebay.....I see them all the time, you might get one really cheap. You won't need a 3" chamber for SD, but I think most of them come that way these days. You also might want to research "some" 870 barrels on your Express receiver, there may by a mismatch there......I dunno.
I would not cut a longer sporting tube and then intend it for SD. In today's liberal society, in the legal fallout after a shooting, I would not want to create toehold for some lawyer in civil court that you used a "sawed off" shotgun on his poor client. This is not just me, Mas Ayoob thinks the same way and has put in in print. A factory SD barrel intended for the purpose, or better yet a slug barrel, intended for hunting, cannot be so easily twisted into murderous intent.