man those look great. you should do a detailed step by step of the actual process and some of the different techniques on how to do those ridges and whatnot and how to do the molding in the sand. It would be great to use for a base rock with a perfectly flat bottom that you could tie natural live rock into so it sits nice and solidly on the bottom of the tank.
Plus even just painting them natural colors and throwing on some fish safe clear coat on them for a more realistic imitation reef system in a FOWLR
would be cool and those people who are artistically inclined could most definitely make them look better then the cheap fake things you buy from a big box fish store or even most local fish stores.
You could even let your kids go nuts with painting/molding other things for their own tanks. Even a simple freshwater setup could benefit from this. Kids like making things like that and it'll promote them to have an interest in something other then the television.