Make it loosely formed. You need all kinds of nooks, crannies and caves for the hiding type fish. You can usually stack it so that it will not fall over, but if you just want to be sure you can use silicone or superglue gel to attach them to each other. Try to make it natural looking, not all stacked up on the back wall. If you set some of it out from the back and slowly stack it towards the back wall it will create openings behind the rock and cause the stack to sort of lean towards the glass to help the stack keep from falling over.