I have been buying used backpacks to experiment with the best ones for lead solo and stumbled upon what might be the best.
The "Camelback Octane" is ultralight (350 grams), thin and stays lined up along the spine, is the perfect size (adjustable as well) for a few different rope sizes, has a great coefficient for rope feeding, and works as a rope bag and even main pack for TR solo sessions where the rest of the cargo is worn on the approach.
This thing has been perfect. I may modify it with a big grommet / eyelit for the sake of an end tie-in instead of knot to prevent accidental feed-through but other than that it is perfect.
They are expensive new but worth it. I got this for $10.