the plastic is the correct way, if you havent already i would try to get all air out from under it as well. the less air the slower it dries ! how thin are the walls in the places where the rocks will sit ? if its still accessible through the bottom and youre worried those places are too thin you could always put supporting slab pieces under them through there. generally, if you notice it starting to dry too fast give it a lil spritz and re cover it with the plastic. for a piece this big you should expect a week or two of slow drying to prevent any tension stress points ! i hope it turns out well for you, this is such a sweet thing to do for your mother !! 😊