If you put just a Halo in the housing wouldn't it mess with light output? Since we don't have projectors the light is reflected off the entire housing. If you put a foreign object in there I would think it would mess with light output.
Well first off, are u looking for projectors and halos or just the halos? U can purchase a it for just the halos if thats all ur looking for on ebay for fairly cheap. Just search CCFL angel eyes and plenty will pop up for you.
If ur looking at projectors and halos in one then u may want to get the projectors i have... they are about $100-115 shipped and have halos of almost any color and are a fair price and work great!