Well, it's almost a story telling itself..
2 pieces of plywood, a hinge, a padlock furnishing and 4 pieces of iron ( which can be wood as well ).
You'll need to buy the hinge at first, as it determines the minimum thickness of the board, as you'll see below.
(Click for fullsize)
The eye-screw, you see next to the hand-hole, is there in order to make it possible for you, to attach a single-cuff (handcuffs) as you can be very certain, that if the hole exactly fit MY wrist, it will almost as certain not fit YOUR's..!