Diagonal cut Menger sponge

The [Menger sponge] is a fractal object that has any number of interesting mathematical properties and one unexpected and very attractive physical property, which [George Hart] explored in a [video] and a page on his [website].

[Menger sponge]: http://en.wikipedia.org/wiki/Menger_sponge
[George Hart]: http://www.georgehart.com
[video]: https://www.simonsfoundation.org/multimedia/mathematical-impressions-the-surprising-menger-sponge-slice/
[website]: http://georgehart.com/rp/rp.html

There have been a number of Menger sponges on Thingiverse, including [one] set up to show the diagonal cut that Prof. Hart discusses. I could never get that part to slice correctly, so I made my own in OpenSCAD. That said, my second-order version gave Slic3r some problems as well, so I've also included the STL after being fixed by Netfabb Cloud. For either first or second-order, you'll need two copies to make a full cube.

[one]: http://www.thingiverse.com/thing:25791