Make sure RHS seam is oriented away from view.
Existing tub is about 1600×1600. Headboard will be mounted in front of tray. Internal rails will sit inside side rails, so we could go with 1500 lengths of steel. For the basic tray (2 side rails, 5 internal rails, some bumper stuff):
50x50x2.5 1500 8x = 2x 6 meter length cut in quarters
75x50x3 1500 4x = 1x 6 meter length cut in quarters
150x50x3 1500 2x = 1x 3 meter length cut in half GALV
Metal yard charges for each cut. Full lengths are 6 meters. Either cut in half and put 3 meter lengths on roof rack, or cut in quarters and put 1.5 meter lengths in trunk.