Mohammed, can you verify that those numbers are actually Tesla and not millaTesla? 5 Tesla is quite a strong field, for instance the magnets used in the Large Hadron Supercollider are around 8 Tesla.
Looking up what I can about the magnetic fields produced during aluminum production shows far less levels. This is off the Alcoa website about a new super smelter coming online in Saudia Arabia:
Four pot ramming machines capable of achieving high, homogenous density paste for the lining and relining of pots. The electric pot ramming machines operate at 460 V/60 Hz and are designed to handle a maximum magnetic field (100 Gauss).
100 Gauss is 10 mT. In case the field actually is multi-tesla I think you will have to get an enclosure custom made out of Mu-Metal from somewhere like Magnetic Shield. Another option is to use an MRI camera, like the one below, they can go to 7 Tesla.
Perhaps they would just sell you the enclosure.