Alright, I though I would contribute to this thread for more than just a contest. For those of you who want to make a 3D tunnel opening, here's how I did it in Idiosyncrasy #13.
First, we start with a blank background like so.
Then we take the mouth of the opening and place it where we want the opening to be on the panel.
Finally, make the ground above the opening and move the nodes so that they fit onto the opening like so.
Tip: Make sure you slant the ground like that and to provide a straight path on the top exiting the opening while being connected to the other part of the path. You may need to practice with this somewhat.
I hope this helps some people in the future.