No matter what you do, in reality, they are not effective. For that matter, cleave gags (my favorite) aren't effective either. I think the cleave gag conveys the impression of helplessness and silence better than the over the mouth, or "detective" gag, but that's a matter of personal opinion. "Mark McCain" from "The Rifleman" was kind enough to demonstrate.
When I was young, we played games and made challenges that involved being tied and gagged. Since the gags we saw on TV worked (or appeared to work) we had a rule stating that you could not talk while gagged, nor could you push the gag out with your tongue. If you did (without good reason), you got tied up and gagged some more - usually twice by each person present when the "infraction" occurred.
Drawscore