I don't use or recommend so called "bioactive" enclosures for most herps including ball pythons -- though I do recommend naturalistic enclosures, which is a reason I'd not use foam ('bioactive' means different things to different people, so I may be thinking something here that you're not). I also question the utility of most backgrounds, at least as they're often constructed (since they take up enclosure volume without a corresponding increase in terrain that's usable by the animal).
If you're looking for a thin (that is, only decorative and not climbable) background then I suppose you could use just about anything that enables the look you're going for. If you get clear on what function you expect the background to have, that can help narrow down the material options.
All that said, if I were to construct something with the qualities you mentioned and is also climbable by an adult BP I'd personally use cork bark -- rounds and branches, mostly -- and I'd use stainless steel bolts to attach the pieces to the back wall. This works with any drillable enclosure (so, anything other than tempered glass).
I tend to get cork like this from Pangea, by buying a bulk box, but the more appropriate sizes are sometimes out of stock (like right now). Larger reptile expos are good places to get cork since you can hand pick the pieces you want. Getting the right pieces of cork can take some time.
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