On my commuter bike I have the Topeak MTX system, and I love it. It does all I need exceedingly well. However, there are days where I will want to ride the race bike to work with a backpack, as I'll have a race or club ride after work, and I may not have schlepped in enough work clothes in advance to ride it "naked". So I'm looking for recommendations for a good, snug fitting backpack that will stay put when riding in a pretty flat racing position. I won't need a lot of room, just enough for a change of clothes or two. No heavy weight, no hydration system. So far the only recommendation I've seen here is for the Momentum. Any others?

Thanks in advance.