Take one of the 576mm 2020 extrusions and insert 4x M5x8 button head screws. Thread on 4x M5 T-nuts enough so they don't fall off.

Insert the T-nuts into the channels on the back face of the front of the picture. The distance from the bottom of the 2020 to the top of the 2040 should be 45mm(blue line)

Take another 576mm 2020 extrusions and insert 4x M5x8 button head screws. Thread on 4x M5 T-nuts enough so they don't fall off.

Insert the T-nuts into the channels on the top face of the bottom 496mm 2040. The distance from the back of the 2020 to the front of the 2040 should be 60mm(blue line)

Repeat the previous 2 steps to add the cross support to the top