Going Bioactive have some questions.

[distaff]

My king likes to burrow. I used regular broken down leaf litter from the back yard, and sheets of bark, also from the back yard. Living soil needs to be light and fluffy, so the sheets hold up the soil. Coir, and frog moss, forest moss, or spagnum moss will help keep it airy. Peet (can work too). I make sure mine is OK for animals. Josh's Frogs and New England Herpetelogical are my sources for the stuff I purchase.

IMHO, a drainage layer is necessary, but some keepers say they do fine without it. What you do NOT want is a layer of mucky soil on the bottom, and this can be hard to avoid if you are going to keep things damp enough for soil organisms to flourish. Hydro balls, or gravel can work. Both get heavy. I prefer sheets of pond filter 1" thick, covered with fiber glass screen. The filter is a matted curly plastic, is lightweight, and can be cut to size. A drainage layer covered with screen allows the soil layer to breath from beneath.

I use dried grasses, dried whole leaves, more dry moss, and more bark on top.

A UTH stuck under the floor will not provide a hot spot for this arrangement. You can sandwich one between two tiles and seal with silicone, and place that IN the tank, but be warned that is "off label" so to speak, you are on your own for safety. (It does however, in my experience, work well.)