I knit a "sock" for my Kindle too!
I sized mine to just fit over the standard Amazon cover --- to keep it closed safely when I toss it in my bag, and add a bit of padding. It's knit to a darn tight gauge, so it's not going to protect it from a tall drop, but it should buffer some bag shuffling.
Mine just slips on with a hemmed top so you have double thickness fabric around the top to hold it stiff and make it easy to pull on and off.
I have so many different yarns I can use, I was thinking of making more.
If I did it out of a self stripping or patterning yarn, that could be really cool.
I was debating stitching a design on the outside... some type of character. Perhaps a Kanjj symbol with a motivational meaning.