If you are in an area where local rocks are scarce, I suggest going to look around at a landscaping rock place. There will be numerous shapes of rock which will fit and at a cheap price. They are used to selling by the ton but may even give you a single rock if you look proud to get it! Think of brick shaped for the sides and one more flat for the "roof".
My small rock cave for mbuna:
I do an overnight soak in a few gallons of water and a 1/2 cup or so of bleach then rinse them and lay them out in the sun to dry. It never pays to assume that there is not grease or oil from the machinery they use. Kills any bacteria or fungus also just to start out knowing it is safe.
I created one for my tank and used aquarium silicone to glue them together, that way I can grab the top shelf to lift and move the whole setup to vacuum. If any overflows just cut it after it dries until it does not show. I'll try and post a pic later.