I bought one of
these solar chargers recently (there are about three American sellers that sell the product via eBay), and it works pretty well.
The build quality isn't spectacular, but it works. But I have actually wanted a more elegant, built-in solution like the one you proposed, and posted a thread on Reddit some time ago about it. The trick is that any solution (including one where the solar panel is integrated into the cover) is going to require a wire running from the solar array to the USB port.