I finished fabricating the back of the case, which consisted of drilling mounting holes through the motherboard trays and PSU mounting plate, through the back sheet and into the case frame. Holes that went into frame were tapped to #6-32 and those in the middle of the sheeting had PEM nuts pressed-in.
Thanks. We decided not to paint the inside in order to reduce cost. An unexpected bonus from that decision is the striking contrast between the black and bare aluminium.
Turned out awesome Rich, I agree with voigts, looks like it was made in a factory & nice bonus contrast due to budget, does look nice.