I bought this one in pink for mine when I first got it:
https://www.amazon.co.uk/kwmobile-Ca...A2N58YCCQNSUNR
It fits really snugly so feels sturdy yet lightweight, the cutouts are exactly where they need to be and it's perfectly compatible with the sleep cover option.
I also bought an envelope style one similar to this but not by KWMobile to see which I preferred:
https://www.amazon.co.uk/kwmobile-Ca...5&sr=8-15&th=1
The one I tried made the reader quite bulky, so one-handed reading wasn't comfortable, and it kept 'slipping' so that when I had the charger plugged in, the case kept slipping down and dislodging the charger instead of staying in place. But, as I say, it was not KWMobile so theirs may be better quality.