As far as I know, no one makes high quality optical filters for camera lenses.....at least I’ve never seen them. Lens filters usually incorporate optical coatings to reduce image degradation, but even then there is can be some degree of deterioration in image quality in some circumstances.
So anything short of a true lens filter would likely have a terrible affect on the image quality a phone camera produces. For instance, putting screen protector material over the lenses would be disastrous I’m sure. Your best bet is a case that is thick enough that the lenses don’t protrude when you set the phone down.