I had originally built the scape outside of the tank. I carefully made all of my measurements, and I methodically glued it all together.
I brought it upstairs to put it in the tank after the glue had cured . . . and it didn't fit! It must have shifted ever so slightly after I had glued it.
I had to take it partially apart, and rebuild it in the tank.
It isn't 100% what I had originally, but it is pretty close.