I went with 3/4 in. which they sell at Home Depot, bundled up in plastic and it's pretty inexpensive. The depth was perfect, just had to cut the lengths on some pieces for my 40 gal. I also put it underneath since I don't use a uth. (My hides which are black plastic serve as uth's it seems. Gosh they get warm,very warm. I have to keep an eye on them with my temp gun. And set my thermostat's with them in mind as well as the ambient.)
I love the Styrofoam . Agree with all you just said about the heat, electric use and humidity. The pieces I put on top I wrapped up with saran wrap. Wrapped tightly around and taped. I put 3350 silver 3M tape on the edges which touches the heat light domes to keep the saran wrap safer from the heat of the domes ( they don't get hot enough to melt but I figure better safe than sorry.)
When i first cut the pieces I had also used the 3M tape to go around the sides of the Styrofoam. To keep pieces of Styrofoam from getting everywhere and then I used the tape to hold the styrofoam in place against the glass. (But there might be a better tape to use out there. I have to keep pushing the tape down to stick to the Styrofoam pieces.) But it does stick to the edges of the Styrofoam great for some reason.