I use vermiculite. I use rubbermaid food storage containers (the rectangular ones). I use 100 grams of vermiculite, and 400 grams water, set it into the crickets for laying for 24 hours, and then remove without adding more water, seal the lid on it, and cook it in my incubator at 84 for about 10-12 days.
Sometimes crickets like to die in the nestbox, or loose a leg, etc. That is usually the source of mold when I do have mold (which is rare- I carefully look for and remove any cricket bits before I seal the containers).