Possibly cases that use elastic to hold down the corners will work. I have one K2 case that I can use with my Kobo. The Kobo is 1/2" smaller in width and 3/4" smaller in height than the K2.
My Bobarra case works okay with the Kobo because it uses 1/2" elastic at each corner and my K2 is a snug fit. The Oberon case is too big and the M-Edge case I have uses the hinge. If the hinge points are the same on the K3 as the K2 then current cases should work.
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