I don't know if you guys agree but this could be the Daddy of all gags. This gag has a cushioned panel with rubber stuffer piece attached. The strap goes behind the back of your head and buckles at the front. And if that wasn't enough, you are also able to padlock the buckle so even if your subs hands are free they can't get the thing off. More time for them to say MMMPPPPFFFFHHHHH! Pics of the gag are below, including a
few of me wearing my own I purchased a few years ago.
I purchased my Lockable Gag from here - MEO