I have also seen them created with just card stock or poster board.
This was an idea that I found on Pinterest from Kathy Griffin:
I've been asked the drill bit I used and the measurements. So, as an update I took some pictures! (I'm a total visual learner!) The drill bit that I used was 1/4" drill bit to create the hole. Here's the picture help:
They are about a thumb width apart... don't worry- an actual ruler measurement is in the next picture ;)
They are about a centimeter apart.
And about 16 centimeters apart..
Do you need more math ideas? Check out these daily math questions for interactive notebooks!