Poultry nipples

I haven't yet heard of anyone installing the nipples on a metal bucket, but it might work. Since the nipples are plastic themselves, it would probably be pretty easy to ruin the threads. You would probably want to experiment on a similar piece of metal first. And also use aquarium silicone sealant to fully seal each nipple inside and outside.

Perhaps you already know this...If you use a galvanized metal bucket, be sure never to put anything acidic in it, like ACV. It will react and denature the galvanization and will lead to rusting.

Perhaps someone who has done what you want to do will pipe up for you.