Reptile Forums UK - This recently updated thread shows some great step-by-step instructions for building a rock background out of foam, grout, and paint.
Custom Foam Background
Caudata.org - Updated pictures of a member’s completed H.orientalis tank after building a background using foam, grout, blackboard paint, and epoxy to make it look like stone. Also see the progress of a nice custom vivarium that this member is working on.@ Caudata.org