I had a FWB years ago that enjoyed a special sort of play we called "Bitting Up".
She wanted me to bit her up with the horse bit that “pony girls” like. I put the tall leather collar around her neck and buckled it up. I had her put the rubber covered bit into her mouth herself, hold it, while I buckled it tightly again behind her neck. It’s about 3/4 “ in diameter and sits well back in the mouth between the molars. Forces the mouth open, and draws the cheeks back when properly placed.