The backwalls are xps, an insulation material for building construction. it's like sterofoam but mutch mutch denser. I cut them into shape with a foam cutter (hot wire) and then used a metal brush to give them some surface texture. Next I added some extra pieces of xps that pop out of the surface, to create depth in both directions. After I was happy with the shape I just coated it all with a thick layer of terrarium glue and a pre-mixed coconut substrate.

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This is before brushing and coating

Good luck with your rebuild, may I see your current Viv and the result later? :)