This is a hollow version of the impossible heart, or actually 2 versions. I needed one for a wedding gift. Off course the happy couple wanted some financial support for their honeymoon. But I have to much pride to give them just an envelope. So we decided they would have work for it. We put the money inside the heart. It took them a while to figure it out how to open it ;)
I made one entirely hollow heart and one with walls. The one that is entirely hollow can only be printed with support. I found it was difficult to put it together with the money inside so I made another version which had inside walls. Now I could stuck the money in the separate parts.
You can use the openscad file to make your own versions. The hole in the middle is made with a scaled down version of the original shape. So a hollow Cube, Sphere and Heart will work. But for example a donut would not work. In the openscad file the size_hole_inside reflects how big the inside hollow shape is. You can use it to change the outside wall thickness. When it is 1 the hole is the same size as the whole product so you would not get an outside wall. If you do not want inside walls you set "withwalls" to false. The wall thickness is the thickness for the inside walls. I also put some code in the comments which put a B and a P in pieces 1 and 2. I included a P and a B stl, so if you happen to need a P and B you can make your own STLs for the heart-pieces.
The STLs for the heart-pieces are without letters.
For the STLs the one with walls is bigger as the one completely hollow.
I printed them on a Prusa mendel. After removing some small irregularities on the edges they fitted very well.