Can I cover the back of a piano?

Does anyone know if it is O.K. to put some kind of nice looking board or covering on the back of an upright piano so that it can sit facing a front window and the unattractive back opening of the piano won't be visible from the outside. I wasn't sure if the back needed to be open for some reason -- sound quality or something else. Also, could the sun or the hot window area damage the workings of the piano? Thanks for any advice.