I've eliminated substrate in all my hisser's boxes (Gromphadorhina portentosa, G.grandidieri, G.oblongonata, Elliptorhina chopardi, Princisia sp.)... only eggcrates and three little dishes (petri) for dry food (pond sticks), fresh food (veggies and fruits) and water crystals...
Only half of the box is heated.
They're breeding a lot, growing fast and happy....