not my site but:
http://ifihadabikeblog.blogspot.com/...lebar-bag.html

you can find the designs for a handlebar bag on the internet in a google group (i think it's the ibob one).

i would add that decaleurs are somewhat tied to the bag/frame size. for me, the VO bag + my 25" frame does not even come close to being able to use VO's own decaleur. i mean not even close.