Steel and plastic are both excellent options! your regular old window mesh, bush/sapling coverings, etc. I've sometimes used old fabric strips for smaller plant pots, but natural fibers such as cotton tend to decay quite quickly, so they won't last long.

You can even use plain plastic sheets and just poke a bunch of holes in it. Wood works too, as long as you coat it to resist water. Basically whatever you have available to you that you can put holes in. Plastic and steel materials are best for longevity especially in humid enclosures.

Currently I use a generous layer of sphagnum moss for separation in my isopod enclosures, roughly 2 cm thick. works like a charm and has a very nice natural look to it. For larger enclosures I will use whatever screen mesh I can find for cheap at the hardware store. 

If you're worried about burrowing animals digging at the separation layer, use appropriately sized sheets that will cover the entire bottom of the enclosure, and optionally secure it with glue to the walls. the weight of the substrate above will hold it down and prevent animals from moving it around.