Ortlieb messenger. It's the largest single compartment bag I could find. Very comfy too. Just a big space to throw things in. I find it way more useful than multiple compartments. Ortlieb make a few similar smaller ones, and the 'messenger' can be harder to find, but worth it.

Edit: despite being called messenger, this is a back pack, so may not be ideal for you, but one to consider for the future