Neil,

I like the design. Is there enough wall thickness of the collar (8mm ID X 10mm OD) to accommodate a small screw?

What about a collar that has a thicker wall and a little standoff, like this... gives you more meat to tap into.

Click image for larger version.  Name: Collar Idea IMG 1.JPG  Views: 1061  Size: 109.2 KB  ID: 6047

This could also allow for a more tortured path on the bottom collar to keep dust out.

My last comment is to not forget to think about how you will assemble this... go through the steps in your mind, sometimes I design something and find out I can't assemble it because there are inaccessible features! :(

Adam