Dunno how to describe those hard plastics... but they can be found in the art section where they sell papers and boards in sheets... and you can find them in different colors as well. the sheets of this plastic material is constructed like cardboard boxes but translucent. You can cut a piece and use a piece of tape to hold it over the flash.
I think i know already. Its the coloured Corrugated cardboard right? The one that school kids always use for projects and stuff?