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Looking for a tall but narrow trunk bag or box that can hold a 14" laptop, most aren't tall enough, yet there are models that are wide enough to let it sit lay in the bag like the Topeak MTX office. I don't like the feel of a single side mount pannier plus they can make you 6% slower at 28 km/h :S:S:S https://www.cyclingabout.com/speed-difference-between-panniers-bikepacking-bags-aerodynamic-testing-results/
Something like the Racktime YVES below would be optimal - unfortunately it's not long enough needing to be 33 cm inside (my rack is racktime compatible, but for other systems like Ortlieb's you can often mount adapters).
Racktime has a trunk box that's big enough for the laptop if it's laid flat I believe, but allegedly it's garbage plus too wide to be aero ;_;
It's a beaut
Think I found it 
What about the Carradice SQR bag? Doesn't need the rack, is about the right size from your spec above and should sit mainly behind your bum and thighs so fairly aero.
May not be quite what you were thinking but someone on here highly recommended them a while ago. Or go totally old school and use a long flap saddle bag. Should be fine if you put the laptop in a poly bag.
https://www.carradice.co.uk/bags/saddle-packs-sqr-bags/super-c-sqr-slim
Got the ortlieb bag and the laptop fits (just barely), hard to close however with a janky velcro closing system, and not practical to carry around with no real handle (it does come w a strap).