Not a bad idea, would also conserve water and possibly warm the medium up some by "solar heating" effect. Might be hard to water though, unless there's hoses under it. Personally I stick with perlite, reusing it over and over. After the first few uses it gets a fair amount of dead root material in it, the small bits after pulling out the clumps. That acts as a moisture retainer, much like peat or coco, but you don't need to buy it, it's free. I let the containers sit for 3 weeks between uses, enclosed in garbage bags to kill off any fungus gnats that may hatch in there. The roots break down somewhat in that 3 weeks time. So you can actually have an organic medium without using soil or peat/coco, because perlite is a natural stone and cannabis roots are of course also natural. If too much root material builds up in the perlite you can always dump it into a container of water and wash it off, the perlite floating to the top. Once you get a good amount of roots in there it holds enough water for a few days, whereas pure perlite would dry out quickly and also not hold the plants up as sturdy. Kind of like growing plants in styrofoam pellets, way too light.