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.