If you are using the now famous Home Depot desk lamp that uses that bulb, the problem is that there is no room for any kind of reflector. The bulb barely fits in the lamp head, with little room around it. If that is the case it probably doesn't matter what you use for a reflector.
If you do have room, then a good ultra white paint is a good reflector, so you could take the bulb out and paint the refective surface with that. I doubt that white plastic is nearly as reflective as that paint, but *********** coat may be pretty good.