How about something about the size of a 55 gallon drum that would stand on short legs (just tall enough to slide a small bucket underneath). A dump in the bottom would allow feed/grain to be put into a short bucket.
Or better yet, an adaptor for a 55 gallon drum that would mount to a hole cut into the bottom for a dump. Again, short legs for sliding a small bucket under. That would require only lifting the bag an additional foot for loading. Because the grain was being removed from the bottom, feed would always be rotated and fresh (no need to completely empty the drum before refilling).
We've got the same problem Pete does. Smaller critters don't eat that much, and nobody really makes anything cool for them. So, rubbermaid and the plastic barrel people get all the business.
Steve