Xsens supplies 3D models (STEP/IGES) for integration purposes.
The models for 4th and 5th generation MTi 10-series and 100-series are attached to this article. Refer to this article to determine which generation your device belongs to.
Next to these drawings, detailed drawings in the MTi User Manual show outline dimensions and screw hole diameters.
A library for the MTi 1-series can be found here.