Costco used to sell the small “Ciao” roller bags that are designed to fit under seats. Anyways.. I have that and it fits perfectly with just enough room to stick a water bottle in front of it.
Holds all my junk and works as a good day trip bag too.

I don’t know if Costco sells it anymore, but it was only $30 and have had it 3 or 4 years now.