The arrow was supposed to be pointing to the airgap between the two bits of extrusion where all the load will be transferred through two thin plates. I lack paint skills.
You want the bits of extrusion to clamp to each other face to face not cantilevered up on thin plates.
You don't need S355, fixtures need stiffness not strength. Better steel gets you a better yield strength but the stiffness will be the same.
That price will go down if you add more bits. Small orders are priced WAAY higher to account for set up and office time.