Step 1: Supplies
A Credit card sized item
2 magnets (not too strong) (or velcro)
A paper clip
Step 2: Cut Out the Poster Board
A rectangle, about 1 cm bigger then the dollar on all four sides.
A thin rectangle, about 1in thick, and about 1 inch longer than then half of the above piece
Some random pieces of poster board, smaller than half of the original piece, but larger than the magnet.
Fold that large piece of poster in half, hamburger style.
Step 3: Magnets!
Put some tape over it.
Lay the random pieces of poster over the magnet, and tape down. (these extra layers help protect credit cards)
Tape up the folded piece on the top, and one side, leaving just one of the sides untaped. (credit card slot)
Lay a magnet on the end of the long strip of poster, making sure that the outward facing side of the magnet is attracted to the wallet. Tape this on.
Tape the long strip, around the back of the wallet, so it folds around and can attach to the other magnet.
Step 4: The Paper Clip
Bend it into a shape as shown. You want a flat base parallel with the wallet, and part of it to slide into the card slot. One you have it like this, secure with duct tape.