A very inexpensive one is this one:
I've not used it, but it looks interesting and it doesn't cost all that much.
Years ago I attended a wedding workshop by a photographer who was selling a kit to make your own out of cardboard. He used it over a White Lighting head. At the time I bought the kit and made one but have since lost the item and the instructions (this was in the 70's). I remember it worked quite well. Perhaps someone will remember who it was.