Aluminium cube
Light sheet aluminium, in this case 22 gauge or 0.71mm thick is easily worked, and while impractical to solder, lends itself well to pop riveting as a fixing method. Scoring with a laminate trimmer blade and a straight edge on the outer surface relieves bending stresses and helps form the sharply folded edges.
An alternative to riveting would be to use double side tape; underrated, yet immensely strong when applied to flat, smooth, clean surfaces.