We're glad you've enjoyed it so far, and definitely, we're thinking about how to introduce and help the player to create the shapes they want. We don't want to make too many constraints for the player though, so it's an interesting task.
The pencils will currently only work if the piece has been hardened, indicated with smoke when cooling from a hot temperature, they are quite finicky at the moment though, as they are concepts of tools we want the player to have. Currently, it works sort of like an "airbrush" holding it near, and pressing A or B should work.
We're looking forward to the next iteration! :)