To fix the see-through sky wall, you'd have to leave an opening above the roof of the cave rather than bringing the sky all the way down to touch it. Then the other side of the cave wall would be visible.
Currently you have is something like:
wall
v
| | |
| | sky |
| | |
| ceiling
What you'd need is:
| sky
| |
| | <- wall
| |
| ceiling
