I use plastic eggcrate and something like PVC pipe to elevated it above the bottom. I put screen on top and cover it with the substrate. If you want a water feature, say, in the front, make the false bottom smaller to leave room for the pool and allow the screen to overlap the side of the structure. For flowing water, I place a powerhead pump in the space beneath the eggcrate and run the tubing up to where the water is coming out, whether it be a drip wall, waterfall, or stream. Just make sure to design it so that you can get to the powerhead without having to tear everything out. I know that many folks here and on other sites have provided step by step photographed instructions.