You don't say what size coal you require - the coal used on enqines is large, heavy and dirty!!
I would suggest seeing if you can get hold of some insulation foam board, such as that used in wall voids etc. This can be cut into irregualr sized lumps and painted black, most builders have an excess of this after jobs and it ends up in skips. Again if accuaracy is not an issue why not use standard childrens' building blocks painted black?
Nick Ellis
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