I keep a very deep substrate in most of my cages. I like to give my snakes the ability to burrow if they want and I also have real plants in some cages so it helps keep them in place. Where I have the uth the substrate decreases to a thin layer with a hide over it.
There is no reason you can not save the substrate for later.
I use a mixture of cypress, eco earth, and repti bark. I mix it to the desired look and keep it in a large tub with holes in the top to keep it from growing mold. When Im ready to use it I add water, stir, and dish it out.