I like my Patagonia Atomic Stealth. It does exactly what you're asking for plus a little more. I was looking for the exact thing you are now looking for: As small a pack as possible; one that would carry a large flybox and a light rain jacket. With a Patagonia Deep Wading jacket (size med) and a large flybox, the fit was tight, but workable (It will not fit a Simms Guide Jacket). It has a few other bells and whistles, the most important was an exterior water bottle holder that gets me easy access to drinking water. I also like the pocket on the shoulder strap. I keep a file, hook hone and abel tool in it. When my fly ticks a rock in the winter, or I get a knot in my leader, I have easy access to those tools. Good luck. There are a lot of nice slings out there now.