I think this is the best design I've come up with yet. This one is square but it'll still work on a round. The back is hinged with a piece of ribbon that is then covered with clay, except for the tiny seam left between the top and bottom which allows it to move. This allowed me to run the necklace right through the top of the bottle, and although it looks like the necklace is then coming thru to loop around the bottom bead, it's just a separate loop made of the same material. I like this because it hangs perfectly straight and there's enough room to actually put something small inside the bottle, and for the fact that anything inside the bottle will stay there.