The hat is smallest 3D structure, but very useful. It combines three planes into a fourth marking an equilateral triangle on that plane.
Since the uses of Space Struts in actual construction of human sized dwellings and structures, on earth or in space, will require many miles of red-tape to be digested. For that reason, a provisional patent has been applied for, and the use of that patent only allows crafts for education, not construction. Let the Patent Pend.