A third order 3D Hilbert curve with 1mm thickness & clearance. 2/8 blocks are rotated so that it forms a loop instead of an open curve. See https://www.shapeways.com/model/111039/tiny_hilbert_cube_variation__1_5mm.html for a 1.5 mm version and https://www.shapeways.com/model/109525/ for a 4th order version.