Some of you may know about this trick.
But I was thinking I wanted a bolt hold open of some sort while clearing my gun at the range. It finally occured to me.
Stick the case neck of a fired cartridge into the ejection port between the top of the chamber and the gas piston, just ahead of where the piston scews into the bolt carrier. Viola', instant bolt hold open device.
Now I know no one cleans their AK, so it won't be useful there, but you could use it at the range when it's time to clear weapons with the bolt back.