I got a new transparent cover for my phone, a Nillkin. It's amazing. Fits perfectly to the buttons/holes and is very thin. It also protects the usb-c port so that dirt doesn't go I there.
I had another one before that made the phone double its thickness almost (not literally, but that's how it felt). I have a OP3T btw. My girlfriend has a Galaxy A5 1000 and had a Nexus 6P and she always uses Nillkin as well, always perfect.
They are hard to find in Europe (we found the galaxy and nexus ones once in Lithuania for 10 each, bought), but it's easy to get then online (eBay shipping from within Europe or aliexpress).