Thanks to @concretedog , @neil & @foosel, I've ended up buying an Xteink X4. Picked it up from ebay from a UK importer, and it came with a screen protector and mag rings, but they didn't have the cases.
I 3D printed one of the slide-on ones but it just doesn't feel nice, so I've made a fold-over magnetic case mostly using some resources I had lying around. Had to buy a small amount of felt for the lining and 3D printed some strengtheners to form the spine and protect the front.