I had one of the Choate folding stocks Ivan mentions above on a 1980's Ruger Ranch Rifle. I thought it was pretty good actually, at least mine was, with a solid lock-up when open, though length of pull was a little short. Never tried it prone though. Might want to look around the interweb for one if its cheap enough. $280 sounds like a lot for a stock, but if you like it, why not - its only money...
Poor 80's scanned pic of my Ruger below with the Choate stock, next to my old HK-91.
Larry