A derivative of http://www.thingiverse.com/thing:19468 . My printer didn't print it (the balls didn't stick to the bed and printing with raft was no good either), so I had to change it a bit.
And as I don't have SketchUp, I made a openscad design that had the same dimensions as the original and added some sort of support structure there.
It is usefull either for bragging or to include it as a part in your thing.