I considered the trunk bag option but opted for a single pannier instead. I'm a high school administrator and in the summer the kids and teachers are gone so we're very casual - shorts and a polo are generally OK.

Occasionally, I've needed to look a bit more presentable for doing interviews or meetings. I have found that a single TransIt DX from Performance holds a pair of khakis, work socks, a white undershirt, and a polo shirt (all rolled) along with my small planner, a belt, and a thermos of coffee. They're quite spacious and the price is right!

I'm not sure how/if I'll get a dress shirt in there when we start up in the fall - I know it will fit, but I'm not sure about the wrinkling... I'll give it a try, but I also have my eye on this garment pannier from Performance.

Good luck and let us know how it goes!!