This isn't a very complicated answer, nor is it the best fix but... If you line up just one of the screw holes on the fan with one of the screw holes on the case you can usually get it to mount, but it will only screw into 1 hole. It could rattle or make extra noise, or may not fit at all depending on the case width.
I've done it to my own build with a smaller fan into a larger fan space.