I know little about modding, but this seems easy. Simply use the cake programming, but change it to have a cobblestone-like texture and make it heal no hearts. This would allow you to make breakable walls, breakable trap doors, etc... But you'd also have to make it so cakes can float in the air first. (They break if you try to make them float.)
I'll make it. It only needs the rock, right? Also, does it need to be right clickable or if it needs to generate as a structure I have some issues with that, but its nothing worse than the jungle temples and pyramids, or the redpower2 volcanos
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