My sister wanted some stencils to replace her paper ones that kept getting cut accidentally. She gave me her paper ones (the white square and triangle) and I re-created them in Tinkercad and printed them in black (1mm thick). 

Then I upgraded the square to be a 10cm square with an 8cm square cut out of the middle. I also added a label to the square and decreased its thickness to 0.6mm.

The triangle is 10cm x11.2cm x11.2cm and 1mm thick.