I saw one on ebay that looked like a speedfeed witness protection only it was wood......never seen any others for sale.
Probably a custom item.
Pretty easy to make one using a beat up old wood stock you just need to figure out how to resize the hole so it will except a shorter bolt.
I'm going to do mine by gluing in a wooden dowel that has been machined for the bolt but it would be easy to fill the hole with a metal epoxy and coat a bolt with release agent to form a mold.....