Your best bet is to head to a REI or any hiking, backpacking or outdoor store ... So as to not appear totally unhelpful, Osprey makes several clamshell options in that size range that many recommend.

Firstly, the tomtoc Travel Backpack 40L is designed with a large capacity, ideal for those who travel frequently or love spontaneous weekend getaways. With dimensions of 20.47 x 13.39 x 9.06 inches ...