I considered putting this in the AR accessories forum but these are only specific to something that has a Picatinny rail. I chose here because it is sharpened steel. I'm also not normally a fan of vertical grip pods, but this seems to serve more than one purpose. I imagine having an M9 bayonet fixed along with this as a vertical grip would be the kind of redundant that only people who wear belts and suspenders (like me) could appreciate.
http://www.gripknife.com/
https://www.instagram.com/p/BX0xdnPF5Rs/?taken-by=gripknife