I did GCSE technology back in the day and designed a collapsable chair. It was hard coming up with various ideas as I had an idea I wanted to do. But no I needed a bunch of other ideas to show design progression.
So here are my thoughts about feed bins I've worked around a few good and bad! A good bin will have the following characteristics:
1)easy access should not be so high that you can't reach the bottom easily, the lid should be secruable so you don't ahve to hold it open while you try to scoop feed.
2) the materials should be rodent proof
3) materials should be sealed so they are not absorbant
4) materials should be cleanable with disinfectant
5) should be cleanable, maybe a tray in the bottom that you could pull out and rinse off, that way you could brush the sides clean they would fall into the tray and then you could pull the tray out and clean that. This would be similar to say commercial rabbit or poultry cages.
6) bin should be designed in a way so that feeds can be kept seperate.
I would say that you need to decide who your bin is aimed at, the sinlge horse owner who will only have a few bags at once, or the commercial yard where they could be feeding 20+ horses. This will effect the size of the bins and other characterictics. For example a single owner could keep the feed in the bags and then the bins would not need some of the criteria laid out above.
Hope this helps.