3DPS Frog has a holder directly above the center of mass perfect for your desk tree.
Native Formats: F3D (Autodesk Fusion 360, filesize: 72.6 MB)
Exchange Formats: OBJ (OBJ, filesize: 24.4 MB), IGE (IGES, filesize: 27.7 MB), IAM (Autodesk Inventor, filesize: 18.8 MB), SKP (Sketchup, filesize: 31.1 MB), STL (Stereolithography, filesize: 78.8 MB)