I have a Field Notes cover from Bellroy. It is awesome, and I’ve had it for years.
I haven’t used it a lot since COVID started, but I suspect it will get more use as things get back to normal.
It is leather, but it doesn’t look like the kind of “grandad leather” a lot of leather covers look. Other than that it looks very clean and simple, it fits a single field notes, it has some magnets to help it stay shut. You can fit a small pen or pencil in the cover.
And if you want to hack it, it is possible to fit more notebooks than the one. I have used it with three notebooks by some clever paperclip hacking.