These are an adaptation by Dave Fils of Dan Erlewine's trick of using a long beam, cauls, and a clamp to gently assist a neck when tightening the truss rod. Dave had the idea to make cauls to fit the Stewmac understing leveling beam. With the cauls fit to the T shape of the bar it is much easier to align and keep in position.
Dave made the originals out of wood and then brought the idea to the 3d club where we recreated his design in Fusion.
The zip file contains cauls with 1.4" and 1.8" string spacing. The Fusion file is parametric allowing for different spacing options.
Dan's video demonstrating the technique.