Rimstar.org has a fantastic project for makers of all ages, assembling a simple pinhole camera from a single sheet of poster board – he uses a sheet measuring 21 7/8″ x 28″ for example.

Here’s how to make a pinhole camera from a single sheet of poster board. I came up with it in order to teach a bunch of kids how to make a pinhole camera and how they work, so I needed materials which would be inexpensive and which I could find in abundance.

Poster board is available at art stores and stores with an art or school supply section like Walmart. Most dollar stores and discount stores also have them and they cost only around $1 dollar (US) each.

Ideally the poster board should be 21 7/8″ x 28″, and the one I’d developed this with was. However, I later bought some more and they were 21 7/8″ x 27 3/4″, so 1/4″ shorter on the long side. I show below how I handled that in case it happens to you too.

Read more here.