It's not the cheapest solution (~40€ on Amazon, I got it for 30€ as a Market Place offer), but it fits like a glove and gives the perfect feeling.
The outside is the same as for the famous Moleskine Notebooks, when closed it actually feels like one of these. There are no Magnets involved, it is held close with a sturdy rubber band.
For storing the pen I got a Leuchtturm pen loop. They are available in various colors to match the sleeve. There's enough room for a 2nd pen on the inside, e.g. a Staedtler Norris or Onyx one for different purposes.
In my opinion it's the perfect sleeve for the reMarkable as it gives it also that 100% paper feeling when closed.