The bearings are much sturdier as the fan is bolted to the motor, and the motor is a generic one meant to be used in any orientation. I personally bought [this](iLIVING 20" Wall Mounted Shutter… https://www.amazon.com/dp/B0BVY8VCNZ) one for my house to cool it in the summer and at half power, every open window in the house has a stiff breeze.
Box fan bearings are not typically designed to run in that orientation. It will probably run for a while before failing prematurely. Might make it a couple months or just a few weeks.
This is a much more robust fan and they come in various sizes to better fit your opening. They move a lot more air, and many styles come with a thermostat and speed controller. Sorry for the Amazon link, this is just the general style: https://a.co/d/bLeBCeD
I look for high endurance drives which are intended for use with pi’s, dash cams, etc. they tend to come in smaller sizes due to the endurance requirements.