I built a cave for the Everlasting Server Community Base. Definitely not as difficult as you might think.
In that one, I built inside the base, not above the plot. I saved the exterior above the base for "phase II". 🙂
In the base editor, search for "Cave" in the search box. You'll find a number of cave rock items that are long and flat (the exact names escape me at the moment). Those flat objects can be used as floors, rotated to use for walls or ceilings. Just place one then fiddle with the F5 button to where you will want to place it (eg. floor, ceiling etc.) and move it into position. Items like the Cave 1 through Cave 5 worked well for walls.
Takes a bit of fiddling but it isn't hard to do. I recommend watching the video guides @Dacy posted for pro tips on placing and manipulating objects.