I am currently using an osprey 26+6 backpack. Not the most comfortable carry, but works for me. I can fit a weeks worth of clothes without expanding. Like the clamshell opening.
In the past, I have used an Ogio Metro backpack. Similar capacity, or even more. Lots of pockets and organization. No clamshell opening, so I typically used an Eagle Creek garment folder to easily slid the bundle of clothes in and out.
This fucking bag is a damn champ. I have done weekend packing enough for a family of 3. 2-3 weeks in Europe solo. Several extended car trips. And over 100 airplane overhead stores. Still looks almost new. Just wish it had a water bottle holder, probably only improvement I would make