We have aluminum corner feeders: https://www.countrymfgstore.com/caalcofe.html
They’ve been in place for close to a couple decades (replacing buckets, plastic corner feeders, etc.), and look like new. They’re expensive but bulletproof, and I expect them to be lifetime feeders. The expense has been well worth it - would’ve saved money if we’d bought them in the first place.
Not hard to clean, either. I normally take a couple dishcloths to the stall, along with a bucket of water with a tiny amount of liquid dish detergent (such as Dawn), soak the cloth thoroughly, scrub out the feeder, repeat with a fresh cloth (not putting the dirty one back in the water).
If the feeder is really dirty, I slosh a little soapy water in it, let it sit for a few minutes while I clean the next stall’s feeder, then come back and wipe out the residue.
To rinse, I get a fresh bucket of plain water, wipe out the feeder with another clean cloth dampened with only water, repeating with another to make sure any trace of soapy water is gone. (In the tackroom, I keep a drawer full of dishcloths, washcloths, and hand towels that no longer make the cut to be used in the house.)
Sounds more involved and time consuming than it is; the more frequently this is done, the less time it requires (can get it down to a quick swipe with a damp cloth).
However, my horses don’t receive soaked feed, sticky feed, feed with oil poured on top, etc. They do get Guiness in the hottest part of summer, but they lick up as much as they can (since they love the stout), and the feeders wipe down easily after they’re done.