I've "rebuilt" a few of these with mashed down felt by peeling up one side of the felt on one side and then sticking it back down again a tiny bit closer to the light trap, which effectively puffs it up a bit closer to the one on the other side, and makes a more light tight seal.

The felt you can buy at most craft stores is not real wool felt, and it's not very light-tight. I found some usable 100% real wool felt online to repair one that was completely torn up at the outside edge, from someone carelessly jamming film into it over the years.

Sure, brand new ones are cheap, but I hate to throw away something that's perfectly usable other than a little bit of felt!