Recommend slim case to keep dust/particles out of iPhone 7 (Jet black)

I have the iPhone 7 Black, and I just ordered the iPhone 7 Plus (Jet) Black. I always used a case with my phone but I still have all kinds of scratches and spots on the back from particles getting in. I would like a case that doesn't allow any particles in because, from what I understand, the Jet Black version is even more prone to scratching.

Thank you