I have seen very old Native American flutes encased in gut. Almost like being encased in rawhide. Helps keep the flute together and probably somewhat moisture proofs it (the outside anyway). You could probably do the same with a bow, sinew the back and when all done and tillered, slide it into a gut encasement and let it dry. Shrink Wrap! Waterproof the sinew. That's similar to what I read they did with snake skins back in the day too. Slide a whole fresh case-skinned snakeskin onto a sinewed bow and let it shrink to dry.Sealed belly and back.