I got the Sony standard cover for my Sony 300. I do wish it had some way of closing it securely for when I have it in my purse. I made a fabric cover for it with some fabric scraps & used a button & ribbon closure. Works, but I find I really prefer the cover "naked" - what I like about the Sony cover is it's so lightweight, like the 300 itself, and I do like that it has the magnets that keep it closed when it's lying on a table. My current solution is to use a narrow stretchy headband to keep it closed when I'm carrying it in my purse.
And yes, the Oberon covers are lovely - I have one for my Kindle. Thought I'd like one for the Sony as well, but they're a bit heavy and I'm realizing I don't want the added weight.