Artistically a splendid model! However, the .stl mesh has some problems: Mesh has 3 non manifold vertexes and 23 faces are incident on these vertices, according to Meshlab. Therefore when you want to perform some changes to the model, like hollowing out a big version, you first have to repair the model, by Mesh Inspector, which is the only application capable of complete repairing. Is is not free, but all free applications fail in this respect.

easy to print!