Better version by themetavirus https://www.thingiverse.com/thing:4521137

Untested: I do not own a 3D printer and therefore cannot test and revize, please if you print leave any complaints or suggestgeons in the comments

Uses four 4-40 x 1-1/4" (or longer) screws and some locking hex nuts. You first need to use the printed parts as a guide for drilling two 1/8" holes into the top of your Triad, and then you can just attach put it together so that it clamps down onto an available rail segment.