I use an Oxford bag similar to this. I like it. No way to really secure any valuables in it so plan accordingly.
Hence the shoulder strap.

I have a tankbag (should arrive today) that has backpack straps that tuck away. Gonna be awkward if I have to carry them both around with me.
I may end up putting that Bestem top box on after all, if I can find a luggage rack on which to mount it...