Emmerson Gray, https://www.emmersongray.co.uk/ a UK-based maker of felt sleeves for tablets, laptops and phones, has released a model specifically made for the ReMarkable:
Colours available are: Grey, Yellow, Orange, Green, Pink. Pen loop is optional. Prices GBP 11.49 without, GBP 13.49 with pen loop, including free UK delivery. GBP 5 for international delivery.
Having bought the initial prototype, I found it to be slightly too small and lacking a charging hole for the USB connector.
I contacted the company and they confirmed they had not actually seen a ReMarkable, so they were grateful for the feedback.
They then very kindly sent me a new one which included the charging hole and was made slightly larger in both directions. The new design is now a perfect fit.
The felt is about 4mm (over 1/4“) thick and very good quality.
Second happy customer!
I bought one of these too and was really impressed with the price, fit, and customer service - highly recommended!
Third happy customer!
+1 on this. Sleeve has charging and grapping hole and feels nicely soft. With this, my reMarkable even fits in the pocket of my winter jacket (it didn't with another sleeve).