It shouldnt take much to shield the pump, look at shielded home speakers for inspiration. They operate at varing frequencies, and at high field strengths, yet require very little metal sheet to ensure operation near TV's. I have noticed that the shielding that is used also runs parallel to the relative poles of the magnet structure on speakers, ie the magnet in a speaker has its pole axis in line with the movement of the cone, and the shielding is placed as a voice coil would be, except on the outside field of the magnet.
Also, if you believe that the need to ground the shielding that you create for your pump, I would run the ground wire to a splice in the pumps ground, nearest to where it enters the pump assembly. If the pump has been modded to draw power from the power supply, I would mount the shield ground to the ground connection that is soldered it accept the plug receptacle, this will help reduce ground loops.
But I am unsure whether this process is needed or not.
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