Updated Gridfinity 6mm Magnet Holder Box

While I loved the improvements that Zenius made to the original magnet storage bin by gabriel.poleze I wanted the bin to sit on top of the base plate, and sit flush with other blocks that are placed beside it. I kept the same internal dimensions but adapted the outer shell to fit more in line with other gridfinity parts. As a result this remix is only for the box itself, as it is 100% compatible with the cartridges found on Zenius' model page.