I can tell you how I made mine.
Mine is made from 3/4 inch plywood. It is 11 inches tall, and the top is a 15 inch square. I have 3 3/4 inch boards that I use on the top to adjust height of the box.
My box is a 5 sided affair, on days I am not squatting, I use the upside down box to store belt, wraps, chalk, whatever might be sitting around, then dump it, turn it over and start to whimper on squat days.
If I were to build another one, I would make it broader and deeper across the top. I think making it 18 inch squared on top is probably about right.
Some people use other methods and build their's out of 2 by 4's, but after I built my power rack base, I had some scrap 3/4 inch plywood sitting around, so I thought what can it hurt? And it has turned out to be a pretty good box. Plenty strong too.
B.