This is a mimic of a much more expensive tool sold by Park tools to remove headset cups out of bicycle frames. Theirs is all pretty and made of high tensile steel, but whatever.
Find some tubing. Copper, steel, even an old flat handlebar will work. Just don't use aluminum; it's too brittle.
Step 1: Cut your tubing down the center
Cut down about 5 or 6 inches through both sides. A hack saw will work too.