Thanks once again, everyone! This is great advice - I can order a duffle and try it out and return it if it doesn't meet my needs. Ed's comment about it banging against one's legs has me curious, so I want to make sure that won't be a problem. I have until September, so that will give me plenty of time for trial and error.
Another reason I want to limit my bags to carry ons (my other one will be the aforementioned canvas courier bag) is kind of embarrassing to tell you about: It is so that I can keep the use of those airline carts to a minimum, because one time I was on an escalator (this was after I was diagnosed, so I have an excuse of sorts) and, when I got to the bottom, forgot to let go of the handle you squeeze to make it stop. This caused quite a ruckus and yelling, so I don't want to take a chance on that happening again.
Lastly, and I keep coming back to my courier bag because that little thing (11"H x 15"W x 6 "D) really does hold a lot, and I've had it with me the last two times, so I am looking forward to travelling light, yet still having all I need.
Again, thanks so much for all the advice.