Check milsurp stores in your area, or do a search for 870 accessories. Gander Mtn. and Cabelas used to sell them, they may still. I can't be entirely sure because all my shotguns have butstocks, and I have never checked.
Just a thought, see if you can get someone to custom make one for you.
If I have to explain why I ride, you wouldn't understand. Explaining sex to an alien would be easier than explaining the feeling, one that boils all that is going on in your life down to what really matters each time you are in The Wind. All I can say is, Life is not Life without it.