Here's how I mounted my Dremel 200 router. Something similar could be used with other rotary tools:
I made this out of 2 pieces of polycarbonate, but MDF, PVC, or aluminum would probably work. Also, I put a saw slit in the middle and used 6-32 screws to mount everything.
For a full description see: http://www.jcopro.net/2011/08/01/a-dremel-tool-mount-for-the-zen-toolworks-cnc-router/