A magnetic field is spherical in shape, holding it over sensors wont get you anywhere. What you can attempt to do is line a back with a few layers of foil and then stick what you're stealing inside of it. A certain member on MPGH got a certain gaming console in such a fashion.
Your going to get sued if you shoplift best buy. Meh if the "Oh look a guy is shoplifting over there" part goes wrong kick him in the balls. That would slow the guard down for a sec. Are you stealing a 6ft tall dildoo?