@chrisaldrich Would this work? www.rickshawbags.com/travelers... ? Not a "notebook", but it would carry a lot of index cards. I think it would fit 4x6.

I use their Diplomat (https://www.rickshawbags.com/diplomat) to carry a Field Notes notebook, my smart phone, and some fountain pens and pencils. It's too small for 4x6 cards, though.