Under a contract with the GRANTECAN, the Commissioning Instrument is a project developed by a team of Mexican scientists and engineers from the Instrumentation Department of the Astronomy Institute at the UNAM and the CIDESI Engineering Center. This paper will discuss in some detail the final Commissioning Instrument (CI) mechanical design and fabrication. We will also explain the error budget and the barrels design as well as their thermal compensation. The optical design and the control system are discussed in other papers. The CI will just act as a diagnostic tool for image quality verification during the GTC Commissioning Phase. This phase is a quality control process for achieving, verifying, and documenting the performance of each GTC sub-systems. This is a very important step for the telescope life. It will begin on starting day and will last for a year. The CI project started in December 2000. The critical design phase was reviewed in July 2001. The CI manufacturing is currently in progress and most parts are finished. We are now approaching the factory acceptance stage.