eco earth, lots of it, and then dump a gallon or two of water in. works great for mine and it hold the humidity for weeks. i also keep 95% of the screen top covered with very think plastic trash bag ( cut open and layed out )

a note on the water, how much water to use will depend how much eco earth you have, i have ~3 bricks in mine, and it takes a good bit of water and mixing to get it all worked in so that the substrate is nice and damp