Magnetic tables contain banks of high tech powerful permanent magnets that are arranged held by a mechanism. That mechanism can be either a simple lever or a rotary control. The action is to switch the permanent magent bank either into a magnetic shunt of soft iron or into the table pole pieces. When the permanent magnets are in the shunt possition there is little magnetic holding force. Some remaining magnetic effect is is normal in the 'OFF' position dure to magnetic remanence in the table's pole pieces from the strong magnetising in the 'ON' possition. The table type of magnetic hold down takes the form of a box and the mechanism is arranged to slide rectangular magnets between the shunt and the exposed table hold down pole pieces. What you are describing is the outer case that forms the shunt and is some form of soft iron with relatively low magenetic remanence that meshes with the bank of oblong magnets in side the device. The parts that are surrounded by some non magentic material are the external hold down pole pieces. The action of the non magnetic separators is to ensure that the magnetic force with in the shielded pole pieces can not leak out into the hold down poles. AND This is important the poles are arranged in pairs N&S to magimise the magnetic attraction to the work piece. Sourcing the magnets will probably be your greatest problem. The magnets need to be treated with care as they lose strength until they are part of a magnetic circuit.
Hope this helps.