I need a design done in an STL file. It is basically just a square, 12 inches by 12 inches with a 2 inch lip all the way around the bottom, Basically, it is going to act as a step. I would like for the "steps" to be able to stack within each other, so the 2 inch lip needs to taper in. So, actually, the base where the lip edge sits on the ground would be the 12 inch by 12 inch dimension, with the top, where someone steps, would be slightly smaller, say about 11 inches by 11 inches. You need to make it thick enough to support up to a 300 lb person.