I bought the iron bear head in Quebec City last summer- it's meant to be mounted on a wall and you pop open your bottle in the bear's mouth. I decided to make an empty sack that you can hang under it, to catch the caps.
I crocheted, from the bottom up, an empty "body" and made a button hole at the back of the neck. When you mount the iron head, there's a button tab to secure under the head. Then you button the body on and once the body is full of caps, you simply unbutton and dump all the caps into the trash.
I tried to make cute little arms and legs- but I was running out of yarn and ended up adding what looks like flippers rather than proper limbs. Oh well, it's still adorable. And functional!