The SuShi septum project aims to realize a high-field, compact septum magnet using a superconducting shield to create a zero-field region within a strong magnetic field. One of the candidate materials is MgB2, a cheap and easy-to-handle material. A first prototype has been tested in a spare corrector magnet (MCBY) of the LHC. Experimental results of this test will be presented and compared to detailed numerical simulations. In particular, superconducting parameters and ultimate shielding performance of the material are estimated, and the observed relaxation phenomena are explained using computer models.