I think this would be a pretty clever idea. I've seen all kinds of homemade suppressors. Car oil filters, PVC builds, coffee cans... even a potato as a one-off. The important thing to remember is; suppressors do not make you 100% silent, even with subsonic rounds(which i wouldn't want to see in the game, anyway). I think this could be pretty viable. From a game mechanic standpoint, they could have degradation system just like every other weapon in the game. If we could get 10 rounds through a makeshift suppressor and have it as loud as a window smashing, that could add a really interesting element of strategy. You could clear out an indoor area of zeds without making the entire neighborhood cranky. Also, bows and crossbows are not exactly silent either. There's a twang from the bow that, though isn't loud, is noticeable. Not to mention arrow impact into something other than a soft target can be heard as well.