Most build a box like Wenchris has, I would suggest making one of the 4x4 sides removable slats, like a guillotine. This provides easy access to the bin once you have emptied out the coal directly against the slats.
You just leave a small gap at the bottom to shovel it out, remove the slats as you remove the coal against them. Once it get to the bottom you can get in the bin to get the rest of the coal. Otherwise you're going to be crawling over the top all the time. For easy delivery make sure the roof is on hinges or can be removed.