Ortlieb Backroller Classic, yellow--but I'm biased.
Waterproof is hard to beat. The contents don't even need to be removed when washing the mud off.
FWIW- I'd use the hole in your rear dropouts with a spacer to mount the rear rack. I've used spacers on my front racks. The spacers actually come with the racks. Steel spacers of various lengths can be found at most hardware stores. It should not take too much to clear your seat stays. IMO, better than P-clamps.
P.S. I just checked my wife's bike and I used spacers to mount a Tubus Cargo rear rack on her bike. She uses just rear panniers and a rack bag. Her rack has been on there for 4 years and 10,000 miles with no problems.
I even thread a thin nylon washer on the bolt. It protects bike finish and keeps the bolt and spacers safe when racks have to be removed when shipping the bike..
