I was thinking of doing a layered mix for mine, starting with clay/sand/gravel mixture base to both store and radiate heat from the heat mat, vermiculite/soil/clay/sand mixture to hold water for a second layer, and then upwards to a primarily soil/vermiculite substrate on the top, leaving small amounts of clay in the soil to offer a digging and burrow forming substrate. Then top the whole thing with hides, moss, etc for decor, water absorption, etc.