Mount corner connectors (mcs_l-verbinder_stabil_60x60) to horizontal extrusions (300mm)
connect 2 of the horizontal extrusions with 2 extrusions (400mm) (forms a rectangle)
add angle connectors (mcs_winkel_stabil_40x20) to support the new struts
connect third horizontal bar to the just build rectangle 120mm from the rear horizontal extrusion using angle connectors (mcs_winkel_stabil_40x20)
add end covers (mcs_endkappe_40x20) to the sides of the foremost and hindmost extrusions