I've used mortar in the bottoms of 2 of my leo tanks, one is 4x2 feet, the other is 3x2. I only poured it about 1/2 inch thick because I still want to be able to move it around without a forklift 
In both of these tanks I used heat tape which I insulated against the wood bottom then poured the concrete on top. It makes a great spot for some belly heat and its easy to spot clean. I just use papertowel where they get on the poop tip and its all good.
Fibreglass would be great to work with for making a landscape, but I'm not sure how long it would take to get rid of the fumes.
I think Candy used styrofoam and wood to make her landscape then sealed it with something. If you emailed her she'd help you out for sure.
Make sure you post some pics when you build it!!
here is one of mine

-----
