There is something I've been using because I have to build so many of these for testing. Just about every hardware store that carries any kind of heating and airconditioning supplies also has sticky backed aluminum tape in rolls of about 25 to 50 feet. That stuff is great for sticking on the back of cardboard to make a reflector. A roll goes a long way.
I've had some trouble with it when I put a reflector together inside the house (68 degrees) and then take the reflector into the garage (30 degrees). The metal wants to shrink and causes the (admittedly very light cardboard) to bend out of shape, but it works just fine indoors. Wifey has a WET-11 template on her computer in the other end of the house thats been there for six weeks with no problems.