There are a number of light-blocking materials. Before you grow anything turn the light(s) on at night, close the door, and see what leaks. There are silver tapes that work, but it depends on where the leak(s) are...along either side of the door or the top or bottom may require different treatment. Thisk black material will work if it doesn't interfere with the door opening and closing. Pardon me while I take another toke...may sound like a stoned response, but it'll work