I notice you cut the notches 5cm apart. Wouldn't it be more useful to cut them in various widths to give more flexibility? 1, 1, 2, 3, 4, 5, 3, 2, 1, 1. This pattern gets increments of 1cm - 12cm.
I tried to bend the thickest sheet possible, 1.2 mm! But it bends with harder. If you need to bend a thick sheet, you can make a similar tool, but use a large square with thick walls.
