I noticed after I took the picture but the linear rail should be over left channel, NOT the right.

Using 2x 2040 jigs, 6x M3x6 socket head screws and 6x M3 T-nuts mount the linear rail to the Y 4040.

Using 2x 2020 jigs, 6x M3x6 socket head screws and 6x M3 T-nuts, mount the linear rail to the Z 2040 extrusions. The gap at the top should be 15mm.

Do this for both sides.