This will be an out of the box idea but, depending how the size of the raster, you could also consider using a Media Layer. This probably isn't the way ESRI was intending to use these layers but your could- export your grid as a JPG or PNG and follow the instructions for adding it to your map. It's one way of adding raster content that might make sense for pure visual purposes over a small area (Media Layers have to be less than 10mb in size)..
