I just received a 6-cell from Dell this week for my 10v. Same problem: very loose, particularly at the left end. Although it is functional, I am thinking of returning it because over time the motion of the battery will lead to plastic fatigue and failure of the latch mechanism. As with other commenters, my 3 cell battery fits perfectly tight.
EDIT: It seems the left latch receptacle on the 6 cell battery is about 0.5 mm too shallow to engage with the lockable chassis latch securely, when compared to the OEM 3-cell battery. I was able to modify the latch location on the 6-cell battery to eliminate this problem. I added three layers of thin, foil, duct tape inside the latch whole on the left end of the battery. The battery is now firmly installed and does not jiggle. The tape is quite durable and firmly attached, but I left a bit of overhang on it so that it can be removed without a trace should the battery need warranty service. When the battery is installed, the tape is not visible.
If it would be helpful, I can post pics of this mod.