Lenses of +.25 diopters are occasionally available in the form of screw-in filters for macrophotography. I bought a Vivitar Fractional Diopter Close-up Lens +1/4 years ago at a photographic flea market, but couldn't fine one online in a somewhat quick search. B&H Photo Video has +1/2 close-up lenses that could project an image on a screen two meters from your window. They are awfully expensive. It is remotely possible that a junked lens could be disassembled for such a low-powered element.