The way I installed mine was to remove the waste board and loosen the 20x20 extrusions that run along the base. Then, I installed the stiffeners and re tightened the 20x20 extrusions. This ensures your lower extrusions are spaced properly and prevents any sideways force from misalignment. Then, reinstall the waste board (in my case, this meant trimming it slightly to fit).
I’m curious about the “torsion box” base you have… where did you get the extrusions?