I have the blue cover and both top corners "creaked" a little more than they should when squeezed. I took the thinnest black rubber band I could find and cut a super thin slice and put them in the corners as shown by the red arrows. You don't want anything that will permanently squish flat, like tape, but something instead that will retain its rebounding properties, like the rubber band. You shouldn't have to squeeze very hard to fit it, and looking at the side profile there shouldn't be any bulge. Thin. Yes... I say, a "sliver-thin" shaving of rubber band. That'll do it!
If this doesn't work. Just throw my advice in the trash. Also, enjoy this picture of my cat.

Edit: After re-reading the original post. I guess my solution doesn't address the cover coming off too easily. It's meant to address the squeeking.