Instead of covering the great stuff with silicone + coco, you can cover it with Titebond III mixed with peat/coco. This is much less stinky, easier to work with, and more durable. The downside is it takes longer to cure, but when it does it's solid.

Building your background outside the viv is a possibility as well. For example styrofoam carved into rock shapes and covered with cement is cheap and virtually indestructible once cured.