I don't have a timbuktu but I have a Chrome Metropolis (http://www.chromebags.com/Messengerbags.html) which is their large messenger bag. I generally prefer the bag on the rack on my commuter but since that bag is smaller than the messenger bag when I have to carry quite a bit I use the messenger.

What I really like about it is the latching mechanism on the Chrome. They use essentially a seat belt type of latch (all metal) and it's heavy duty. The stabilization belt can be used in two configurations as either a waist belt or a second shoulder stabilization strap depending on what works for you.

The bag, is of course, water proof. They use truck bed liner material for the inside pouch and the outside is cordura. The inner pouch is essentially the same size as the outer shell but this two pouch approach keeps seems from being exposed which prevents water seeping into the bag at those points. I can attest to the waterproofness of this bag as I've had commutes of an hour through pouring rain and the contents of the bag were completely dry.

Mel