With any sort of flexible mold + expanding material, you need a rigid outer jacket mold to keep it from radically distorting the flex mold. In the past (casting heads in expansion foam for a Grand Guinol theatre experience (IE, blood 'n' gore) we would fill the flex mold with plaster or something else rigid, then pour a solid jacket mold in plaster.
For a rock-face, which I presume is a one-sided mold and open to the 'back', you'll need a rigid support for the front and then some hard back for it to expand against. For cheap and easy, cover one face of a piece of cardstock with clear plastic packing tape. The foam shouldn't stick much to that. Use more tape to hold it against the combined flex mold/jacket mold piece.
Take care not to let the expansion foam contact the plaster jacket mold - they'll bond irretrievably, and you'll have to rip the foam or chip away parts of the jacket.