Not wanting a messenger bag?
I got sick of trying to fit everything into a backpack and went to a messenger because of the.. Well they're designed for bikes and motorcycles, and latch in two places to prevent swivel, but also sit at an angle on your back, giving you more packing width.
I use the Chrome mid sized messenger..
It has a full seatbelt latch, plus a secondary under the arm fastener.
I carry a laptop, change of pants, bike lock/chain, bike cover.. Various other things, and can just about fit a pair of shoes in there too..
It's completely water proof. I've used it as an umbrella here in the more violent Seattle cloud bursts. It doesn't have the type of waxed coating that wears through. it's solid military strength tarp material.. Reflective along the stitching.. When they go on sale they are a steal.
It would take a lot to steer me away from this brand at this point.