It's almost four year ago that I wrote a piece about the Maplin's Solar panel that I bought in an attempt to keep the battery on my GS topped up. It seemed to have been a terrible failure. It was unclear whether or not it is waterproof and so it was contained in a plastic bag. In reflection, this may have reduced its efficiency.
The optimistic report was http://invictamoto.blogspot.com/2008/03/flat-battery-thing-of-past.html
In a wave of perhaps misguided optimism, I have bought another one. This made by Ring, who make all kinds of automotive and home electrical equipment. It is supposed to be waterproof according to the website.