Necro, from the dead! Muahahaha (The forum reminded me that it's been too long since I've poked this thread.)
Anyway, I made an underground construction tile. The tile numbering is goofy. There are three original tiles: #377, #445, and #445. Yes, there are two #445s...so rather than confuse myself, I used #337 for the underground construction and #445 to denote the above ground construction. Unless Leeor wants something different.
Anyway, without further ramblings...here is the underground construction tile! (Which also reminds me, I need to update the above ground.)