I figured this is where I'd choose to rant first.
If you ever plan to do this yourself, read on and learn from my mistakes.
So I wanted to make my own box with 3 different heights - 11", 13", 15".
So I went to Home Depot to buy some wood and spoke with someone about what I was doing to get his recommendation of which wood to buy and how to build it for best support.
He told me 3/4" plywood is the strongest and cheapest they had and would hold 500+ lbs which I told him I needed without any support inside the box. I got it cut there as well and we agreed that we would need to cut (2) 11"x13", (2) 13"x15" and (2) 11"x15" in order to make the box. So I cashed out at $30 and took it home to get started on it.
First mistake - I never took into consideration each board will stick out 3/4" when I nail them all together.
Second mistake - All sides were the correct height except one. I ended up having 15", 13" and 12 1/2" ((11" + the (2) 3/4" sides)) to work with.
I was very happy with this outcome!