We use wooden troughs, called bakies round here, from the agric store with change from £20 (£10 the last time we bought one) They don't rust but they do eventually rot, if you don't store them inside to give them time to dry out over the summer.
Here, a major consideration is weight, as it's so windy that even the wooden ones have been known to blow a fair distance. Plastic would be gone in a jiffy. We have tried home made ones but without much success. Bakies need to be sturdy as sheep will shove them around.