Schwartz P Surface model with 27 unit cubes. Approximate dimensions are 65mm cubed, with a 1.4 mm wall. Learn more here:
http://facstaff.susqu.edu/brakke/evolver/examples/periodic/periodic.html#psurface

Modeled in rhino using approximate surfacing techniques. Thickened mesh created in Meshlab. Printed at 0.25mm layer thickness without support material.