Thanks guys. So I had an idea:

Looking at this pic of an XS
http://www.timbuk2.com/static/images...p_jb-jb-jb.jpg
maybe I could just put the carabiner on the buckle at the front that actually closes the bag. What do you guys think?

And honestly the XS is already bigger than the bag I use.

Yeah its a custom

Misha