First of all, I don't know if I could choose among them, but my favorites are: butterfly fiol (the most), eurasia and gera. For subjections, I think some clip up and bottom of the cover, to click the device between them. And... could you consider to make them for the two Cybook, Gen3 (I have read again, you mention it, thank you) and Opus. You would have a sure costumer
(they're very similar to my PDA cover, that a Spanish craftswoman made for me).