Cabin Zero

Pros

• Cabin zero worth a look
• Kanken is pretty much a square (cube ish but flattened a tad) so you want something deeper really, right? Check out Cabin Zero.
• Cabin Zero size chart of personal item by airline: https://www.cabinzero.com/pages/airlines-underseat-size-chart-all I would start by determining the maximum dimensions that work with the airlines you want to use.
• The Cabin Zero bags are very light for the size. Do go be dimensions vs volume specs as they tend to be overstated.
• I use cabin zero and have for many years. It's fine as personal item in all sizes if you don't fill it up.

Cons

• Cabin Zero are pretty boxy and simple. I think that could work for you. Just keep in mind that they have a reputation of having the capacity overstated. However they are still roomy.
• I've been eying a Cabin Zero for a while for when I just travel with underseat luggage, but still didn't get one.
• They seem to be ok. Dont have one myself. Biggest downside seems to be that there not the most comfortable if you carry them for long periods
• Yes Cabin Zero overstates their volume specs in general. The bags are well made but basic harness/ergonomics.
• For me, on a short bag (Cabin Zero), it didn't really worked but it did transfer some of the weight on my other bags making it more comfortable.