These are what I use on my 3 board oak fencing:
https://www.bahrsaddlery.com/fortiflex-over-fence-feeder-fortiflex-over-fence-feeder.html
They are not too deep so horses can continue to see while they are eating. I spread them out far enough from each other so they don’t squabble (though the boss pony will eat fast and push out the slow eater, but he can easily see him coming and he tends to finish around the same time anyway as he gets less grain being retired and only 13h in good weight).
I actually use a bungee cord and attach it to the metal hook and then loop it around the bottom board. This way they can’t remove it from the fence, as they have done this and enjoy trying to destroy them on the ground. I also drill little holes in the front corner to drain the water out of it in case it rains at some point. I do wet all of the grain I feed them anyway, so its already pre soaked. They do hold quite a bit of grain if needed.
None of my annoying guys try to get in these little tubs and they can’t remove them now that I bungee them down. I suppose the only danger I see is if they run around and crash into one? It hasn’t happened in all the years I’ve used them, but you know horses!
I never use deep water buckets since they have to shove their heads so far down into them, especially when they are in a group setting and can’t see the boss horse sneaking up on them. I don’t even use deep water buckets for feed ever - in stalls or at horse shows. Just too dangerous. A famous stallion got his jaw caught in one and broke it trying to get his head out and ever since I heard that story, I never used water buckets for feed again.
I actually use these feed buckets in stalls when I go away to shows. One to feed grain in and one for wet haycubes. I love them, but they are not super stable and can rotate and move a bit. So I just use them at away shows.
I use the first feed buckets I linked for outside feeding and use the same for indoor stall grain as well.