the most important thing to a viv is a drainage layer.This layer allows excess water to drain to the bottom, reducing the chance of mold/mildew.
Buy some type of rocks or even marbles, be sure to sanitize them. Lay down about 1/2 in of rocks/marbles. On top of the marbles would b a sheet of mesh. On top of the mesh would be 1/2 in of cocofiber. You could even use natural soil to mix into the cocofiber.
One thing to remember that maintence is key to keeping a viv healthy. Do not over mist.
Another good idea related to a healthy gecko, if you feed your crested crickets, you would want to do this in another container. The cocofiber/soil mixture could cause impaction if your gecko ingest any of it during the hunting process