From the "Make it or Fake it" department, a possible solution. Get ahold of some patterned plastic sheeting. The material mentioned here is only a 12 inch square, but you should be able to find larger pieces if you look.
Cut a circle out of a sheet of black foamcore. Attach plastic to foamcore. Shoot a light at the plastic from way back so as to allow the pattern to spread to fill the circle. You should be able to make a light as big as the largest piece of plastic you can find. You may need to stick some frosted gel in there to spread the light enough to sell the illusion.
I apologize to the professionals here for such a low tech, kindergarten craft day idea. At my level of the industry, if I want something special I have to do stuff like this.
Upon further thought... Get a few of those springy reflector thingies. Angle them right, hit them with something from behind camera. (an ellipsoidal with an iris if you can find one) It could work.