I have found a good underseat bag that is a bargain. It is from a hunting/fishing outdoor store: Cabela's. The bag only costs $12.99, has 6 pockets, and holds about 20 liters. Cabela's Bag. I generally prefer bags without a lot of pockets -- one big opening is great -- and I depend on packing crates to keep things separate. But if I am putting a bag under the seat, I do not want to have to pull the bag completely out or root around in it on the floor to try and find things. With 6 compartments, I can segregate different things: 1) Items I know I will use through out the whole flight (e.g., headphone cable, charger block, charging cord), 2) Items I will use once in the flight (e,g,, alcohol wipes), 3) Items I hope I do not have to get to (e.g., backup charger, backup cords, power bank), and so on.
If you prefer to have a logo with a big mouth bass on the side, Cabela's sister company sells the same bag with its logo: Bass Pro Shops Bag.