Forest Floor cypress mulch and cocohusk are already some of the best humidity holding substrates around. Some other ways to boost humidity are scattering damp sphagnum moss (soak with water, then squeeze out the excess) throughout the enclosure and increasing the size and number of water dishes.

The type of enclosure you use also makes a big difference. Plastic Sterilite containers with holes drilled in the sides work best for maintaining humidity because there is much less humidity escaping compared to the screen top of a glass tank. That said, glass tanks can work too if you block most of the screen top with tin foil.