Recommendations

The Verage Basic Underseater

I love that you can use this as your "base" and stack a backpack with a trolley sleeve over it. 

BoardingBlue Underseater

Works with JetBlue's unusual measurements! Again, love that I can stack a purse or backpack with a trolley sleeve over this. Only two wheels, though. 

Coofay Backpack

Under max dimensions (16.53" x 11.81" x 7.87") for personal items on most airlines. Has a trolley sleeve and opens more like a suitcase anyway. Be careful not to overstuff, but the side straps can help compress it. 

Lovevook Backpack

16.5" x 12.5" x 7.5"

Has a trolley sleeve to stack it over another bag, and I love the "keep open" top zipper. Lots of compartments and more like a traditional backpack if that's what you prefer.