Ok, recent discovery of mine IF you want to simulate just the feeling of having a hard poop in your diaper. (Without actually pooping anything out.)
I recently bought an "acrylic contact juggling ball" on eBay. (I don't juggle, I wanted to use it in a video.)
Anyway, I was wandering around in just a diaper last week and playing with this acrylic ball, it's about the size of a baseball, very hard and kind of cold. Well, I...Uh...You know, got the notion to drop it down the backside of my diaper and, OH MY GOSH...It feels...So much almost exactly like having a big, hard, poop in your diaper. It even feels sticky against your legs and such even though it's not really. Kind of presses against all the right areas down there, though, you know?
Well, just thought I'd share. Stay wet.