We bought a Grandpa's feeder and it seems to be rat proof. The only time we've had rats around the run was when we had a couple of rescue birds in a separate area while the others got used to them and we had a regular feeder in with them. The rats took just a couple of days to figure that out. Once we moved the hens over, and removed the feeder, the rats went elsewhere.
The feeders are expensive but you don't lose feed to the rats and they only need filling once a week.
Mind you, the rats went through 100kg of feed from bags in the barn last year until I realised that mistake and took to sitting out with an air rifle each evening and putting down some bait stations. That reduced the population fairly quickly.