I think they like chopped up human fingers for a substrate.

Na, I'm not really sure but found this.
"Substrate

There different types of substrates you can use for a Tokay, newspaper and paper towels are cheap and easy to clean out the cage. You can also go with a more naturalistic look with 50/50 mix of peat moss and sand. You will want to add a layer of rocks on the bottom of the cage and add the peat moss and sand mix right on top of it. The rocks are for drainage so they soil does not stay to moist for the plants. Snake plants and Philodendron are two popular plant choices for a naturalistic vivarium"