I have these for all my shotguns. They either have too nice of wood to risk drilling for a swivel or not enough plastic to hold one. Don't know if you can get them without the magazine cap. Give the company a call I would think they would sell just the sling.
http://outdoorconnection.com/products.cfm?page=details&cat=Sling&sub_cat=Total%20Shotgun%20Sling&name=The%20Total%20Shotgun%20Sling