This thing can't exist. And yet here's the photographic evidence that it does.
Puzzled? You can print one out for yourself - or check out how it was built, using OpenSCAD, on my blog:
OpenSCAD is a free, open source modelmaking program that's easier to use than you might think.
This thing was designed in OpenSCAD, and you can follow the design process here:
When you print it, it needs to be aligned so that the gap in one of the sides is flat on the bed. After printing, rotate the whole object before you photograph it.