3M command strips should be very strong/secure, while also allowing for adjustment and will detach without residue when the time comes.
I own a 3d printer which makes most of these projects look like the perfect fit for a 3d print (even when there are far simpler solutions). So that is what I'd do :)
Command Strips work similarly to the adhesive that holds batteries into phones. Very strong hold in the directions that count, but pulling the tab stretches them in a way that causes their structure to collapse and release. Must have for anyone who rents a house and doesn't want to put holes in walls etc, but I've also found them to be very useful for all sorts of off-label applications.
They're strong enough to hold heavy picture frames etc on to walls, lasting for years and still detaching easily. I'd certainly trust them to secure this little motor!
.....or just screw into the AC unit somewhere discrete and hope no one notices.
Unless you have made a mess with wires getting out looking like a fire hazard in waiting I don't know anyone who would stop at 2 tiny screw holes. Heck the landowner may not even remember the unit was not made that way in the first place.