I have seen how many people added to lists or collections, you should know the original was here: http://www.thingiverse.com/thing:137367

This is just a remake with a slight improvement

Because all printers have not such precission to make the hole just tight enought to let it get through, this is why at the design there is a small gap, wich it was not included. It actually works better, but

the 3D file is not a clean one... but because I also show how slic3r works,

on Pro tip, when boolean does not work you can play with positive and negative surfaces by the slic3r algoritm, slic3r would take out the part you don't need when you live it open. A contra-intuitive rule of non manifold meshes