Depending on resources ...

Depending on resources available, put at roof over it, on an angle that would be suitable to attach solar panels at a later date. It does not have to be a high roof, just enough for the right pitch. You will still catch all the rain fall as it will run of the roof into gutter into pond. Seal the sides to restrict sunlight. Otherwise make a floating platform from 44 gall drums? join with timber and place a cover over it of some kind. Make peaks in the cover where the drums are so that you create a valley for the rain to run to and drain through, this will also reduce the surface area the sun can see.