Dirt, real dirt, not organic top soil(recycled tree bark), not recycled corn cob, recycled paper, but dirt, real top soil from outdoors, sift the rocks out and clumps. Make it deep, deep enought that your monitor can burrow down underground, and hide and thermoregulate, as well escape you. They need this. You can add a 10% or less amount of sand as a mixture to stiffen it and cause it to hold a burrow better. Sand is one ingredient in a desert, but guess what niether of your monitors are desert dwellers. Bosc monitors (sold in the US pet trade as a Savannah monitor) are semi-arid to arid grassland/woodland dwellers that live in woodland areas as adults, and farmers fields (99% are found there)as hatchlings. Petstores make their money from all of the supplies and worthless light bulbs they sell you, not from that inexpensively priced animal you buy, after all didnt they tell you it needed UV lights, desert sand mix, jungle litter, etc etc? Good luck.