I used red film foil that is used for theathre lamps (in front). I guess it was about 620nm. That means that light lower than that will not pass the red filter and kill your print. paper is only visible to about 560 so if you use a filter above that it will be fine.
I made a wooden box with a 7watt lamp that produces 20 watt. It has a long life and about no heat. I tested it with one sheet of foil distance 1 meter. And guess, the paper was still fogged (i take 10 minutes as a test time because when lithing this is the time needed). When I used 3 layers of foil, it was fine for 10 minutes.
Seems like one layer of foil still has some holes in them which spread the normal light.