I know polystyrene (Styrofoam) and xps foam are generally used and are safe and can be purchased in bulk for cheap.
For the coating on the Styrofoam I used colored sanded grout, and you can also use quickrete concrete coloring to change the color of the grout and just buy it as plain white if you wanna color it yourself 
Got a lot done today so far! Had to make a trip back out to buy more silicone though because I always really underestimate how much I’ll need 
Going to do the rest of the background tonight and then let the whole thing air out for a while, then it’ll finally be ready to set up for Mirage 
Same, oh my god same. XD
This is coming together so well! That textured BG is gonna be so cool.
