Mario block

YOU could make this out of 9 milk cartons, if you will. I didn't so I had to improvise.
Finnish milk cartons are 7x7x23 (ish) so you can get a 21x21 cube easily. Just tape 3x3 fully opened, preferably clean and dry, milk cartons together from the outside (leaving the one in the middle loose). Cut the tops and fold them to form a level side of a cube.
Cut the middle carton heightwise according to your needs. I used a beer can as a measure.
I used a stencil for painting with acrylics. I painted on primed duct tape, which lead to chipping paint after hours of intense use.