the eggcrate used is plastic louver, such as this -->
http://www.goodmart.com/products/lou...tm?source=gsGM
any obstruction can cause "dead" space in an aquarium, but good filtration can always overcome that. the vortex flow created by lily pipes has been the best way for me to have a current throughout the tank.but for hills,
I know some people do put rocks under the substrate (even in nanos, they just use smaller rocks) or most commonly, the hills are built around the rocks in the hardscape.otherwise hills without support, as you know, just level off over time