There's a few bags which fit the United Personal Item requirements
https://www.flyertalk.com/forum/unit...forcement.html
I think I've seen the 5 Cities bag mentioned as quite close to the measurements
and AeroLite 19L
Neither has a backpack strap, only over shoulder.
Europe LCC personal item sizes
Personal item - Ryanair - Wizzair backpacks
A few others which are soft sided and can squish into sizers
MEI Mini Voyager
https://meipacks.myshopify.com/colle...-voyageur-mini
Cabin Zero 28L
https://www.cabinzero.com/products/c...seat-cabin-bag (usually overstates volume somewhat)
Gonex Personal Item (suitcase style open)
I'd be a bad FT'er if I didn't mention the much loved RedOxx Aviators
Extra Small
https://www.redoxx.com/extra-small-a...bag-91013-xs/p
Small (don't pack full)
https://www.redoxx.com/small-aviator-kit-bag-91013-sm/p
Mini Boss
https://www.redoxx.com/mini-boss-91016/p