I've used the obsidian golem models for these, I've just green stuffed the furnaces in the torsos and given them a more earthy look by stippling browns and greens into the rocky texture of the model.
The models were painted to be very similar in colour to the rocky base, to make it look like they are constructed from the stone on the base. The only exception being the sandstone areas on the shoulders etc, this was purely to break up the overall look of the model.
I have another regiment of these still to go, but next up is my steel behemoth, that I should hopefully have finished in the next few days.
