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There definitely are: my local Tractor Supply has a bunch of those and even more are available online (plastic grade ones, that is)!

Ok, wait for others to chime in, but in the absence of better solution and if you don't want to use plastic -> I know that polyurethane coating is used safely in aquariums, so you could try that (you might need to quickly sand a bit the galvanized surface to rough it up and help adhesion) Edit...

Is it a galvanized one? If yes, do you add vinegar (or anything acidic for that matter) to your water?