Sorry, there was a mistake on my side, the stud2 needs to be a stud2a I did not completely edit it, that makes the line go away.
Your bottom quads were ok, but they had the wrong winding, I did not include them, but here's the necessary code (basically your initial first four lines BFC swapped)
4 16 -12 30 -12 12 30 -12 15 30 -15 -15 30 -15
4 16 12 30 -12 12 30 12 15 30 15 15 30 -15
4 16 12 30 12 -12 30 12 -15 30 15 15 30 15
4 16 -12 30 12 -12 30 -12 -15 30 -15 -15 30 15
Gerald
Your bottom quads were ok, but they had the wrong winding, I did not include them, but here's the necessary code (basically your initial first four lines BFC swapped)
4 16 -12 30 -12 12 30 -12 15 30 -15 -15 30 -15
4 16 12 30 -12 12 30 12 15 30 15 15 30 -15
4 16 12 30 12 -12 30 12 -15 30 15 15 30 15
4 16 -12 30 12 -12 30 -12 -15 30 -15 -15 30 15
Gerald