Are you needing a cover to keep fish from jumping or to set a light on, or reduce evaporation? The cheapest after market solution is to get a piece of glass cut at a hardware store. (If this is a full size replacement, I'd avoid the super-thin stuff.) I wouldn't do the foil myself, but I guess you could do Saranwrap if you had to have something. Just make sure it isn't air tight, as you can run the tank out of oxygen that way. I also wouldn't try to put a light over the foil or Saranwrap.
It is a violation of federal law to use this product in a manner inconsistent with its labelling.