I'm having difficulty building and fitting the jaw in my new cheeky boy. I'm not surprised nor am I a stranger to this kind of trouble. I always seem to figure it out, though not always in the traditional way.
I've lined and narrowed the edges of the jaw opening with Magic Sculpt. I tried using paper clay the first time, but that didn't work as well as I'd hoped. But I did use paper clay at the bottom of the opening and that seems to work okay.
The issue always develops after I remove the chin from a casting. I lose material from both the chin area and the sidewalls, and never evenly. I won't be using this jaw for an eventual mold. That was the original plan, but this one won't work. The good news is this go-around has taught me a few things for the next one... I hope.