Prepare the following parts:

1x 290mm (same extrusion as previously).

1x 359mm extrusion.

1x joining plates (in addition to the extra one left after the previous step).

2x M5x10 screws.

2x t-nuts. Slide them in one side of the 359mm extrusion as shown in the picture.

Repeat the previous step to build a 2nd mirrored extrusion + plate assembly.

Check that both assemblies are the mirror of each other and that the extrusions are the same length (359mm).