Here are some photos of my indoor and outdoor setup for my two 1 year old iberas - they are only 5cm long though! I find that the combined heat/light lamp works quite well and the heat diffuses across the table effectively according to the thermometers.
I have just plain topsoil after experimenting with coir, mixing it with sand which made no obvious difference. It is cheap and easily disposed of! I keep moist by pouring on water a couple of times a week aand gravel - which I think is controversial as some suggest they can eat it - though they are far too small at the moment to even get it in their mouths!
https://imgur.com/gallery/1wYd33k