MileHighLife
Blower of glass, grower of grass
Yes, its necessary. Otherwise the current couldnt pass from one battery to the other. I know they sell "magnetic spacers" and they are fairly cheap (I mean its a frigging magnet with the diameter of a pencil eraser)...
However, I went with even cheaper route. I sourced a small magnet from the box that my G-Pen came in (the only legitimate use I've gotten from a Grenco product). Its used as a closure for the lid of said box, and I just dug it out with my fingernail and cleaned the glue off the back. Works like a champ! Ive dropped this shit down a flight of stairs, and no issues with the magnet moving.
I'm sure you can get creative yourself if you have some spare shit sitting around. Heres a shot of mine:
I stack flat tops with no problems ... no earth magnet.