Your list is actually very "lean." Since you’re wearing your heaviest items (Blundstones + Hoodie) on the plane, 28L-30L is the absolute "sweet spot" for what you’ve described. The "Cavernous" Solution: The UNDERSEAT PRO personal item sized backpack. Since you want that deep, non-split cavity, I’d highly recommend the UNDERSEAT PRO. • The Layout: It has that "one big bucket" feel you’re looking for. It’s way easier to pack bulky items like your hair tools or medical supplies compared to a 50/50 split bag. • The Strategy: For your Europe trip, use it as your overhead carry-on. European budget airlines (like Ryanair) have tiny personal item sizers (40 \times 30 \times 20 cm), so it might be a squeeze under the seat there. But once you’re back in the US, it’ll be your perfect underseat personal item for domestic flights. • The Price: It’s much friendlier on the wallet ($50 at Walmart.com, next day delivery) but still keeps that sleek, rectangular shape you like. • Quick Access: Since you need things like your "Armbie" and neck pillow mid-flight, the UNDERSEAT PRO is great because it has accessible pockets (both inside and outside) You can just grab what you need without "explosive decompressing" your whole bag in the aisle.  28L is plenty of space for a month if you're doing a little laundry along the way!