I attempted this project because I thought it was a good way to start building boxes. Anyway I thought it was a little difficult in squaring the project. But after a couple of variations this is what it looks like in the attached pics.
My wife loved them that much that I built 2 square planters as well.
I used the following wood dimesions using a wood called "Weathershield" it was a very inexspensive wood sold at my local DIY store.
2×2x36 (bottom frame)