In theory, you'll be wanting something with a pronounced "ridge", the area that narrows for you to form a seal around, holding it in.
My thinking is that when entering/leaving, the force of you against either the top of bottom of it would push the gap up/down holding it in place.
While a smoother, less pronounced ridge would be far more comfortable from your end of events (I'd imagine?) the less pronounced it is, the easier it is for it to slip out along with you pulling out...
Just my thoughts, I've not tried it.