The 3-M plastic reapir kit i used was realy thin. You are saying that you want to make a clay mold out of play doh or something?
It would be thin and go right into the mold,it would work great to cast a new stud.
Guess cast one and sand it with #80 on both the stud and the part then use more of it as glue to attach the new stud.

Says 3-M auto mix 5883 plastic repair Ez Sand use to repair SMC/Fiberglass reinforced plastics and other ridgid plastics Filler and adheasive work time 10 min. sand time 30 min. cure time 4 hours.
I used it to fix some cracks in the 95 ram 2500 dash and it stuck well. But was thin and tended to run since I was doing something that was verticle