After ordering and returning 8 bags a few years ago, I use an older version of the Briggs &Riley Transcend Rolling Cabin Bag as my "personal item" that goes under the seat. Not sure if the deminsions are the same, but the newer version looks pretty similar to my bag. While it does not have a flat top, I do put my Aeronaut 45 on top using a .

When sliding it under the seat, it actually works to my advantage that the top is curved. So many boxes under there these days. Works like a charm, and I have (knock on wood!) never been asked to gate check either bag with this set up.

Hope that helps.

Rgustafs