Thanks for elaborating, I wondered about whether you used a deep seat for the pavilion/culet to stabilize it laterally. I've done that too in a similar situation and it is a kicker. Cut and fit, cut and fit, cut and fit....
Curious also like Erich, did you set the squares and then add the center section or set them with the center in place? If the latter, what did you use to get the channel hammered down right next to the center or is that why the picture is only from the top?
Just kiddin' around.. .I consider the greatest compliment I can ever hear about my work to be "How the heck did he do that?"