You don't have to use long screws to sandwich the fans in the top, you can buy 6 or 8-32 screws 3/8 to 1/2" long and use nuts on them.
I've superglued the 8-32 X 1/2" screws to the fan frame so they extend outward, and then just used acorn nuts on the outside top of the case too...that way the whole unit is removable without standing on your head and cramming fingers into impossible locations.
You can also drill slightly larger holes in the top of the case and use fan mounting screws. You know the ones that bite into the fans plastic hole to screw it to a case, or mount a finger guard with. Just use them down through the top.
You can get a whole case of assorted screws, including those at frozen CPU. My kit has been priceless.
Edit, if your fans have the fully enclosed corners you'll have to use the long screws though..you know some have plastic cast inbetween the corners so there's no access to the holes from between the frames. Add 5 mm to your fan thickness for the correct length.