Black pond expanding foam works. Spray, let dry, carve fall. Once done and tested can cover with an oatmeal consistency slush made of Titebond III and shredded coco fiber. If using fake plants can use EcoEarth for substrate; but if live plants need ABG type soil.

The egg crate will work to support false bottom. Can also use plastic ties to attach different EC pieces when designing waterfall and pump/filter areas. Lot's of ideas in the members build's with what works and doesn't.