The leftovers are very useful, so don't throw them out. None of my local hardware stores sold this, so I had to order it online. If you are shopping locally, it's helpful to carry a magnet with you to test for magenetism. Just don't carry it next to your credit cards, or other things that are affected :D.
Step 3. Glue on the Chalk Paper.
Magnetic Photo Corners
American Crafts Chalk Paper
Galvanized Tin Step Flashing (used for roofing, but don't get the aluminium that is also used).
Foam Board Black Acrylic Paint