if you're using desk lamps, what kind of lights are you using? are you using compact fluorescents or regular light bulbs? most desk lamps have a white finish on the inside, which is normally good enough.
if you are using just regular light bulbs, switch to compact fluorescents, they should provide more light. and better light for your plants. but as hoppy asks, what kind of lamps are they, can you provide a picture?
I have a desklamp that has the similar bulb that you're using gunk. I'm using it with my 10 gallon tank also. I do not have any reflector either. However, the fixtures do have a powder coated white paint which does have an excellent reflective capability. I'm not sure if you'll need to add a mylar or another type of reflector.
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.
Just to jump in here, hope you guys won't mind... i had two of those light on my 10 planted shrimp tank. I had an old 18inch exoterra light fixture and gutted it as well as the two desk lamp and put the bulbs, ballasts, and clips into the exoterra light fixture, but at any rate, when I took the bulbs out of the desk lamp it had small mirror-finished film behind it. I wonder if this would not have been enough of a reflector and/or if yours just didn't come with that? Anyways, I cut in here because I wanted to ask if anyone knows where to buy that type of paper-thin, mirror-finished reflector? I've noticed that my coralife fixtures have a similar reflector. Sorry to butt in...