Hello Jumper, welcome to the forum. Merry Christmas!
Beware the very significant side-loads on the walls. Coal is pretty slippery, and doesn't 'stack' very well. Take a look at the photos of other's bins here:http://nepacrossroads.com/viewtopic.php?t=441
Another note: shoveling coal from a pile is hard work, especially if you are shovling into a pile of coal. The coal deflects the shovel very easily. [at least stove and nut size does]. But shoveling coal up off of a hard surface like concrete or plywood is pretty easy. The shovel slides under the chunks of coal. So plan on a floor for your bin. Or design in a chute to dump coal into buckets or a wheel-barrow.