Couple of suggestions:
provide the “hand drawn” look and feel.
I would like to give the “idea” feeling, a “work in progress” status.
By having a clean layout it almost feel like it is a final decision.
keep using last selected configuration (if the last square I created was square blue and without filling, make it so that any following new square by default has the same configuration)
I prefer to create transparent containers, every time I am forced to do an extra step to remove the filling.