Please note this is a work in progress. This design should work, however it may change. Video of the holder in action:
plus.google.com/109144216682485717110/posts/NgQP221Byrh

And the final result (this was the first successful print - optimizations are needed on the software side, as well as a bit of calibration and a finer bit for the dremel)
plus.google.com/109144216682485717110/posts/gqBArZNd4Ay

This is intended to link a Dremel Flex shaft

amazon.de/Bosch-26150225PD-Dremel-Biegsame-Welle/dp/B0002SMO36/ref=sr_1_1?ie=UTF8&qid=1359412830&sr=8-1

with a standard X carriage. To use:
1. Print part, and insert two or four M5 nuts into the nut traps inside.
2. Remove the plastic "nut" from the end of the flex shaft
3. Insert the flex shaft into the printed parts
4. Insert entire item into your X carriage
5. Tighen M5 bolts to grip the shaft, and add the nut back to the tip of the shaft
6. Screw into x carriage
7. Mill, very very slowly
8. ...
9. Profit!

Original alibre design uploaded, please change as you see fit!