Yes, that's a big structure, not much to do but wait. However, you can reduce it a bit, I think at least by one layer in X and Y, there should still be enough vacuum in those directions.
Writing a general script for setting up heterogeneous structure that would work well for 11.8 is far from trivial, and that version is becoming old soon anyway, so there is no point in spending time on that. However, the structure you have is suitable to computing the interface between the metal and CNT, so you're in good shape already.