When I build onto something OUTSIDE of the tank I'll use polystyrene foam panels to build on. Makes is much much easier to work with. You cut the foam to fit into the tank for the back and the bottom, keeping in mind the trim, and then connect the two with toothpicks. This makes an L shape that can be built onto. Then the rock casts are piled together and connected. (I use toothpicks for ease of use.) Then foam in the spaces in-between the rocks with GS foam making a single solid piece. The spray foam can then be covered in all kinds of material. Most commonly people use a colored silicone material then dusting coco husk or sphagnum peat moss onto it. From there you can then just drop the entire thing into the tank and glue it in place with a dab of black silicone between the glass and blue foam frame. Make removal if you want to tear down the tank later a breeze.
Nice thing about doing it this way is that you can reach all the areas much easier outside of the glass tank when applying your silicone or other material. You can always cut the design into pieces if you wish like I did for the two "Isomers" build I did there. Also if you wanted you can even plant the piece a bit then drop it in so you can get much better detail and ease of planting when working with small volumes/delicate plants.
Its a really good way of doing it I feel. Lowes/Home Depot both sell equivalents of this polystyrene foam. One has the 3/4" blue DOW foam, and the other a 3/4" pink version. Either works well.
The other nice thing about it is that it can fill the volume and make it so that you don't need a false bottom. Excess water in your substrate will move to the lowest point in the tank and can be pulled out or you can use this to your advantage and make a lagoon or water region. Typically though, you will need to filter this water or remove it periodically as it can stagnate. But less so I feel than most false bottoms do. At this this way the water has access to air so it doesn't produce AS much anaerobic bacteria and the resulting hydrogen sulfide stench.