Wow- all these answers and mine is the perfect solution and is what is done in Europe. You guys are making this way too complicated, especially for something that small.
If you don't want to buy a whole box of them, use small single hole "d" rings and then a paper fastener through the hole - though the backing board, split the tabs on the fastener and tape them to the backing with frame lining tape or something equally as strong. For a 5 x 5 it will work fine.
Paper fasteners like these: A Passepartout hanger uses the same type of fastener but also has a small round ring attached to it that is held on by the fastener.