Draw your desired “key” shape on the cardboard. Use a utility or craft knife to very carefully cut the key shape out of the cardboard.
Trace the key shape onto another piece of cardboard and cut out again, giving yourself two replica cardboard key shapes.
Cut two pieces of elastic at 15". These will be used as the arm straps. Glue both ends of one of the elastic pieces to the inside end of one of the keys, creating a loop. Repeat for other piece of elastic and other cardboard key. Allow time to dry so that elastic is firmly in place.
Dribble glue all over the inside of one key, making sure the glue is on the same side as the elastic. Place the other key on top, sandwiching the glue and elastic ends inside the middle of the two keys.