I take PVC tubing wash it throughly, let dry. Cover the tubing with aquarium silicone and roll in the substrait of your choice. let dry for 48 hours rinse and place in Tank in desired location. You will need a pair of latex gloves because the silicone can get messy. I have also done the same thing to teracotta pots I cut in half, bisecting the pot with a long toothed wood saw. You get 2 caves out of one pot this way. Like I said it is a bit messy but there is a real sence of gratifaction making your own decore.