Default Ultra Slim Cover (not Samsung)

I would advice to ask of you. I've been using for some time now the Ultra Slim Cover Samsung but I've noticed that after a couple of months, not more, part of the cover at the volume rocker is deformed and no longer adheres to the body of the phone and it gives me nuisance.
I was looking for and then another cover, of course not Ultra Slim Cover of Samsung, the same thickness as the original (if I'm not mistaken 0.2mm) that do not give me that problem.
What do you recommend?