The original Secret Heart (44579) was optimised for Makerbot, and just worked, but I had problems printing on anything else. They published the OpenSCAD (thanks), but even after updating for current version and lots of tweaks I had problems, not least of which was that Simplfy3D refused to properly bridge the top unless you make the walls really thick.
I have re-designed this from scratch (twice now), and the OpenSCAD file is included.
The main problem with anything that has moving parts is the clearance tolerance between parts as printed. If that is too small then nothing moves, and if too big then the whole thing is loose. As such I have included STLs for versions with clearance tolerance of 0.2mm, 0.3mm, 0.4mm and 0.5mm. I am using 0.2mm on my TAZ 6.
To save you the hassle of printing the whole thing and finding your have the wrong one, I have also included test prints of key components - the main joint and the hinge for the top flap, each in 0.2mm to 0.5mm clearance tolerance. I suggest printing the main join and seeing if it will turn - you may have to use a knife to separate the two parts slightly to get it to free up.
I have also included an example with lettering, which you can easily make from the OpenSCAD file.
P.S. (12th Feb 16:23) I have been tinkering and re-loading these files a few times...
Separating the parts after printing
Print the heart using ColorFabb nGen