The best Idea for an outside coal bin that I have ever seen is... use an old decomissioned grain wagon.
They look like this http://www.greenfunstore.com/John-Deere ... 45098.html.
Bury it in a hill side that you can reach the top with your truck to fill it. Already has a shute for coal and I bet it would hold a years worth without issue.
I am using a 4'x4'x4' plastic tote with molded feet on the bottom, It holds about 3500lbs of nut. Best part is I have a forklift and I live 30 minutes from the source. I stick the container in the back of my F350, drive to DeRenzo's, get it loaded, drive home and unload the whole thing in one shot with the forklift. (No need to shovel anything) I can stick it in the shop or outside with a cover on it. I have no way of dispensing the coal though. I just shovel the coal for that day into a bucket from over the side of the container. When it gets tough to reach the coal I just go get more
I'll get a picture of it when I get home