I have the exact problem as you. I found that an underseat bag with wheels so often didn't fit in certain planes and was left with a somewhat uncomfortable flight. I resorted to a soft-sided bag and put something hard in it, like a book. I try to arrange the contents of the bag in a kind of pile that I can put my feet on. It isn't as stable as my old computer case roller, but it fits under almost all seats (the exception are the regional planes that have one seat on one side of the aisle; in that situation I make sure to have a seat on the two-seat side). Hope this helps