I didn't say I had solutions not being a sewer at all.
But I did mention Velcro that I've seen applied to readers. It would seem they could sew half of the Velcro (probably the hook part, not sure if it matters) to the inside back cover and supply the opposite loop end with an adhesive back to attach to the reader. That would certainly cover all sizes of readers. But again, I'm not a sewer at all, it would seem a possible solution though. Dropping the reader while reading though is another story and I doubt any cover will protect it then. I read a lot on public transportation, and I usually hold it in both hands because of the unexpected bumps encountered from bad roads, sudden braking, people bumping into you, etc.
When I'm not reading, my 350 is inside a deep pocket, and unless my hands are cold and inside the pocket, it's zipped up, it's impossible to fall out, so the magnets keep it closed enough for my use. But for a fancier/pricier cover, I did mention I'd like some type of clasp, but would opt for a flatter, smoother style that maybe would be kept closed with a magnet (if strong enough) that would flap over, rather than the button type clasps with an elastic loop that I've seen that would protrude and eventually the elastic would stretch out. If it was just an additional leather flap that would fold over the front and stay in place would seem to provide additional protection. Even Velcro on the cover flap would be perfect for me, doesn't need to be a magnetic closer.