Yeah, a single bowline on a cliff is solid, but only if you’ve got a true anchor—like a cleat or a solid ledge. If it’s just a loose outcrop, the knot will still hold but the whole setup feels a bit like a wishy‑wash. As for moss, it’s great for dampening and gives a natural look, but it’s heavy when wet, so it’ll pull the tarp down fast if the wind picks up. Also, moss can act like a sponge—if it dries out, you’re left with a crumbly, dusty mess that won’t hold much heat. A better bet is a thin layer of synthetic insulation or even a piece of old sailcloth; it’s lighter, dries faster, and you can stack it on top of the moss if you really want the aesthetic. So, keep the bowline, double‑check your anchor, and maybe rethink the moss as the primary insulator.