I'll try to help. The problem with the grandpa feeder is the feed tray has no real sides, front lip, or even a feed lip. They put that wire grate in there and that is about all they can do. Back in the day, it was all there was. I suppose the tooling costs are too much for them to change their design.
I'd start off first by tracking your feed consumption, should be no more than 1/4 pound per hen per day. More than that, it is either the feed raking or the rats are getting into the grandpa feeder which is not uncommon according to the reviews and plenty of customers. Figure out what is causing the most feed loss.
What you might do to the grandpa feeder is to start by choking off around one third of the throat of the feeder. Empty the feeder, insert a strip of cardboard from inside the feeder to cut down on the flow of feed. You want to lower the level of the feed in the feed tray to make it harder for the hens to rake out feed. Next, you need to add a small angle iron around the top of the feed tray, sides and front, if the angle iron is at least 1/2" wide that will help a lot on feed raking unless the feed level is so high that it is close to the angle iron. Aluminum might work but the problem with mixing metals is that you can set up a primitive battery that causes corrosion. Lots of ammonia in a coop, salt in the feed, some humidity, you got a battery. So use iron/steel if possible. Home depot will have short lengths of angle iron. Heck, even a strip of wood, 1/2" x 1/2" screwed through the sides and front
I suspect though that big old wide grandpa feeder treadle plate is being overwhelmed by rats. It takes a lot of feed for a rat colony to survive. The grandpa feeders that I have seen in person are balanced so that a big rat can push the lid open. Mice, they usually get in a gap. If you have a game camera or something similar it would be telling to watch the feeder for a couple of days and nights.
If they are pushing the grandpa feeder lid up you can add weight to the lid but they make the lid super lightweight and balance it carefully for safety reasons.
Not much else you can do though. If the feeder is under two years old they advertise a two year guarantee. You paid a premium price for the feeder, double the cost of a better one, don't feel bad about expecting the product to work.