Just came home from my local Performance store. I went in intending to buy a trunk bag to hold roughly what you're looking at. I didn't see a single one that I would feel comfortable putting a shirt and slacks into. I ended up buying a single pannier (this one) that looks like it can hold what I need.

When I have to start wearing shirts and ties again in the fall (I work in a school and we're on summer hours and dress...), I'll probably pick up this garment bag pannier. Seeing it in person, it is really nice.