The device tray is fully mounted, taking 9 FabBosses — 2 on each side and one in the middle. Only the one in the middle took any real careful measuring — and that was because I couldn’t get my MIG gun all the way underneath the tray. All the other FabBosses were bolted to the tray, trimmed to fit, and tack-welded into place. Then I unbolted the tray and fully welded all the ‘Bosses into place. Since they have an .060 wall thickness, setting the welder at sheet metal settings was just right for fully-penetrating, quality welds.
One other thing that the FabBoss does is to keep Dynamat from being sandwiched in the bolted interface. That makes proper cap screw torquing easier — no compressible gooeyness to compensate for.