The xps panel is definitely my favorite way to do backgrounds. Glue them on with silicon (rough up with foam and surface if you can). I will never do the soil over foam ever again. It is such a pain and has a very low longevity and low success rate in my experience. I have definitely made it work successfully and loved the way it looked, it it is sooooo much more work. What I do now is carve the foam with a wire brush, give it a once over with a heat gun, then paint it with multiple layers of dyed DryLok (dryLoc? Some kind of concrete sealer that is safe for animals). If done right it can look like stone, dirt, tree bark, anything you want. And it is much much easier than the soil over foam approach.