I did a fair bit of research before I did the ground mods on my Black Hole (which has the same boards as Haunted House). I tried to find the most unobtrusive attachment points on each of the boards for the ground wire. Most of these attachment points are on the components rather than on the boards themselves. I ran a single wire from the attachment point on each board to a common point on the inside of the head to tie the grounds together and also attached the ground wire from the bottom of the cabinet to this point. This way you do not have to worry about sanding back the cold plate of the power supply to use as the point to tie the grounds together.
Here are the attachment points on each board:
Power supply (left hand side): 20180214_222809 (resized).jpg
CPU (left hand side): 20180214_222017 (resized).jpg
Driver board (upper right):20180214_222031 (resized).jpg
Sound board (upper right):20180214_222134 (resized).jpg
Speech board (Haunted House doesn't have this board):20180214_222155 (resized).jpg
Tying the grounds together at a common point in the bottom of the head:20180214_222231 (resized).jpg