The psu_lower_mount in this step is slightly different for MK2(S)/MK2.5(S) and MK3(S) but they have the same functionality and assembly.

Take the psu_lower_mount and insert a square nut.

Slide 1x t-nut into the lower channel of the rear extrusion, from the right side of the frame.

Assemble the psu_lower_mount with 1x M5x10 screw. Don't tighten fully yet as it will need some repositioning.