Take 2 560mm and 1 496mm 2040 extrusions. Make sure you pay attention to where the holes are on the sides of the extrusions for the M6 screws that are part of the blind joint mounting.

Thread 4x M6x16mm button head screws into the threaded holes on the end of the 496mm 2040 extrusion so there's a gap of about 2-3mm.