To make the square hood, I started with the round one. Here's how I did it.
You'll need a few things in order to make the hood: thick but flexible card, a ruler, a paper knife or pair of scissors, and a pen or pencil. You will also need to have some tape handy, as well as a flat black paint and a brush to paint the hood (the inside should be black; outside can be any color you want).
First, decide how deep you want the hood to be. I decided on half an inch. It'd be good to estimate the circumference of your lens too; for me it's about 5 1/4". After making the measurements, you'd then cut a strip of card; in my case that strip would measure 1/2" x 5 1/4".
You'd then want to form a ring with that strip of card around the lens barrel. The fit should be snug. You might have to trim the strip down to get a good fit. Once the fit is good, tape the two ends together.
You've pretty much made the round hood, and now you'll need to paint at least the inside of it. I used black poster color for mine, undiluted.