I've been using a duffle as my personal item for the past few years, have a few of them. They're all around the size of this tumi one: https://www.tumi.com/p/double-expans...l-01173441041/ (never have it expanded though)
It sits on top of my 20/22 inch rolling carry on, and I've only really been questioned about this setup once at LGA by the people who check your BP before you get in the Precheck line. A swift reminder that she had literally no authority and to call an AA manager over if it was an issue and she let me go. I usually fly F so no GA has said anything so far. FWIW it does fit under the seat in front of me easily, so that's what really matters.