Coco husk should be okay. Most people like to use washed play sand though. Maybe you can do a mix if you want? Some people put smaller laying bins in the cages, and when the female starts to dig they move the female into a larger trash can sized laying bin with 12+ inches of substrate in it. If you decide to do that, put a branch in it for her to hold on to, and maybe a fake plant or small real plant in the substrate (females like to dig near the roots of a plant.) if you use a lid, be sure to cut out a hole and screen it for air flow. You can put her UVB light over the top of the tub for light.