I refuse to buy any cover that has elastic anywhere, including holding a reader in place. It always stretches out eventually (and I just hate the look and feel of the elastic on the front of the reader when it's also used to hold it in place). And for the Kobo, the way they extended the four corners past the reader made it bigger than it needed to be.
I've had original covers on my Sony for 2.5 years now that are as perfect as the day I bought them... they have no elastic.
It's not the "sleep" function that adds any significant weight to the reader, the only difference is the tiny magnet. It's the fact that you have full protection for the reader, including the screen, so I don't mind the extra weight. The neoprene cases usually just fit around the reader and don't cover the screen at all. You will eventually end up with scratches and probably a broken screen in just a neoprene cover.
The 3 covers I've used and can recommend:
Magnetic Ultra Slim Sleep Cover - this is the cover being discussed in this thread and it's my favorite, great price, great protection, although a few have had problems with the correct magnet placement.
Gecko Slimfit Cover - great cover even though pricier, but the flap used to close it bothered me, it's just something I wasn't used to before.
Magnetic sleep cover - great cover if you like that design (the reader will actually slip inside so it really adds another layer to the cover making it a bit thicker than the other), great price, great protection for the reader.