You can try a technique I use for 'flat-work'   .

To make the face of the wall :  Knead up some epoxy and roll it out between two pieces of wax paper to the desired thickness .    When it starts to set ( don't leave it in the wax paper too long ,, depending on the brand , the epoxy will stick and tear out small fibers ) scribe the surface detail you want -- then affix this to whatever substrate you want to provide the thickness needed .

http://www.scalehobbyist.com/  is a good source for textured sheetstock