I was looking for a way to use the Dremel 225 Flexible Shaft while not having my Dremel 4000 buzzing all over the place. The solution was simple: hang the 4000 from its hook and secure the 225 by clicking it in a tight clip. Works with no problem at 35.000 rpm :).

The top holder is a remake from http://www.thingiverse.com/thing:1280556, just adapted it to have the same style.