Miles, matte medium will work (you can thin it about 50/50 with water), but I find that it works no better than thinned white glue, but costs a lot more. I don't think that yellow carpenters glue is the best choice for this type of scenic work, either, as it's supposedly not water soluble after it dries - not good if you need to make changes. It too is more expensive than white glue. Around here, 2 small bottles of matte medium cost about as much as a gallon of white glue, so I always use the white glue. If you need to hold stuff, such as rocks, in place on a steep slope, "paint" the area first with undiluted white glue, then place the rocks or other loose material, then give the whole area an overspray of wet water (water to which you've added a few drops of liquid dish detergent) to help the glue soak up into the added scenic material. By the way, to mix up a batch of thinned white glue, use hot tap water, as it seems to mix faster than cold. And if you're just gluing regular scenic material, such as ballast, wet it thoroughly with a generous spray of wet water before applying the glue mixture. Without sufficient wetting agent, the glue mix won't be drawn into the scenic material deep enough to make a good strong bond.
Wayne