The lion-plate with tap is bought. It just needed painting.
And here is how I did it:
First I've put the clay in place. It doesn't need to be flat. Then I sketched the stones, following the lines of the unflatness, making stones in various sizes. On the right you see the next step: defining and shaping each stone. On some stones I made cracks and bumps. Then the whole wall gets texture by beating and stroking with a firm brush. I applied several layers of paint, but also gave a few stones one layer of a different colour, so not all stones will have the same colour. It needs a bit of variation to look realistic.
To show my husband what I did, I placed the lion-plate on the wall, and it stayed there while drying. That was by accident quite a good choice, as the clay shrinks on drying. With the plate in place, it kept his place. It was quite stuck after a few days of drying!
I still need to add moss, climbing plants, a little bird-nest in a hole between the stones, etc, but so far I am pleased with the result.
Update on my cat: he still sleeps a lot and surely has muscle pain, but he's doing very well. You already said it to me: cats are really tough creatures!