@ Jeffs,
What?!? I don't think that there is any way that my trunk bag is going to fall off, it has two velcro straps and two bungee connection points. I know that it is not as fancy as what some of you are suggesting, but it has been a good durable option for me. Moreover, it sounds like the OP is not looking for huge cross-country bulletproof touring panniers. Personally, I try to commute with as little as possible.