Spatial (external) calibration of the sensors is achieved through a novel registration method, utilizing an easy-to-build calibration structure that serves as a registration anchor.The calibration structure is realized with 4 standardized IKEA package boxes (JATTENE, 600.471.51), of size 56x33x41 cm^3, as well as 32 unique Quick Response (QR) markers  [ISO/IEC 18004:2006], placed at the corners of the boxes’ side faces.

The set of QR markers can be found here.
A CAD 3D model, the exact 3D virtual counterpart was also designed. The CAD model and sample code can be downloaded from here.
A complete manual with instructions on how to build the calibration structure can be downloaded from here.