My kid likes to play with the Hot Wheels monster trucks and needed something to smash. I made this mold so he can make perfectly sized cars out of Playdough and smash them all he wants without destroying anything else in the house.
I used this simple model: https://www.thingiverse.com/thing:21101
And just used a boolean operation out of a solid block and scaled it appropriately.
Smoother the better but since the cars are only made to be smashed 30 seconds later it doesn't really matter.