**Uploaded a second version (all STL's are in the zip file at the bottom). It has thinner walls around the pivot and can only use 6mm Diameter x 3mm Thick cylindrical magnets to stay in position. Otherwise it's basically the same as the first version**
While watching The Illusionist for the first time I was really taken by the scene where Ed Norton's character creates and gives the locket to his sweetheart. I quickly decided to design such a locket, but after seeing a variety of nice designs floating around on the web I pretty much gave up on it until having this idea to create a box based on the same concept - one that can change shapes and only opens when in the heart shaped position. I've been calling it the Secret Heart after listening to Feist's cover song.
I tried to keep all the components printable or easily accessible at average local stores, but there are a couple random components in this version - either 2 small magnets or a spring from a clicker Bic pen along with a BB (or any small sphere about .17" in diameter). The magnets are really the best to use because they keep the box in the heart shape and keep the lid halves together when opened, but the spring and BB seem to work decently if you can't find magnets of the right size. If you do use the magnets then the spring and BB only act to keep the box in the rectangular shape, which isn't a big deal since friction seems to do a good job of it. I've uploaded a few different variations of the lid for this design, each with a different hole size for different magnet sizes, so hopefully most people can find a usable magnet. If anyone has a request for a different magnet hole size let me know as it's pretty easy to change the model and make new STL's.