BUILDING FAKE ROCKS??

Hi, I was wondering if anyone could help me out with building fake rocks. I want to include it with my bioactive terrarium just to add some accent and save some money because I know the type of rocks I want would 1. Be a little pricey & 2. Be difficult to set up to be solid and safe. I plan to make a whole system of basking spots and caves built out of styrofoam for my beardie, Paco, and making more for a new addition who will be coming in the next few days. Now, my concerns are if the cleaners (isopods and springtails) in the bioactive part of this will be harmed or messed with the fake rocks. Secondly, how can I prevent the styrofoam from melting from the multiple basking spots? Thirdly, what would be the best ratio of dirt/sand? I'm thinking about putting 30% soil/70% Play Sand.

If you have any advice, please let me know