I usually fly in F/C but even there overhead bin space is limited and the items you need are out of reach. I really want a rolling bag for under my seat on any airplane. Probably about 14x14x6" deep wheels included. I don't want to use the overhead bins. I want basically a purse on wheels.
I have tried the Travelon quilted so-called underseater and it is way too deep at 9.5"with wheels.
I ordered the Bric's underseat rolling tote and sent it right back because the online measurement said 5.1 " depth and it was 8".
A separate folding cart is an option but I used them all through the 1970's until wheeled bags came out. The carts are quite long even folded up, prone to breakage, and bag slippage. I do, however appreciate everyone's suggestions on here. I am now considering the Victorinox wheeled tote at 7.5" depth.
Let's hope some manufacturer designs a small underseat wheeled bag that they actually road test and measure accurately.