I have
this Gecko cover for my Glo and love it. It's the smallest and thinnest I've seen so far, but it may not be as light as you want it. If they offer any kind of protection from the reader flexing that can break the screen (for which this one is excellent, cover is really solid and well made), they're all going to have some weight to them.
You might also check the cover in
this thread, although I think it will be thicker and slightly larger than the Gecko because of how the reader slides into a pocket, giving it another layer over the Gecko. With the Gecko, it just snaps in place and there's no extra space around the edges like with the official Kobo cover (which made the Kobo cover add too much to the size of the reader for me, so I looked for another option).