I had to tighten down my 4 wide 12AA to 3D quite a bit because something wasn't making contact until I did so. I beleive the flat contacts in one of the head pieces could have been raised a bit more, but no matter an extra turn and I was there.
Some extra springiness on the head tabs might give more flexibility for differing cell sizes, they are recessed and barely more than the level of the plastic with no springiness, In fact on a few of them I can't tell if they aren't the same level.
It also makes you wonder if some batteries are longer than others though as I had 3 tight columns and one loose one, throughout several different combinations, finally just adding the extra turn and hoping everything would be ok.
I think this is par for the course on the bigger holders, I just always err towards caution and didn't want to give it that last turn unless absolutely needed since it might squeeze some of the tighter columns more than I had wanted. It wasn't that tight of a turn, I'm just squeemish with batteries and any new thing I buy.
It seems very well made and I've had no issues. It can handle the need as long as you remember what your working with as MrArtillary says.
EDIT: Regarding the lack of springs on this design, I think it's a great design and just seems to give notice to something not noticed in other spring loaded designs, i.e. battery length varies a bit due to the complexity of the components used and variations in assembly. On the other hand once you have secured your FM pack it is a solid unit that you can plunk in and out and charge forever with no danger of batteries getting lose or spring tabs breaking down (not to mention being bad conductors).
If one of your battery columns seems to be a greater length try moving some batt's around ( I should have used some plastic calipers to see which cell was causing me issues). The batt's are sturdy and can take some pressure, just use common sense. Now that my pack is in use it will condition itself and become a solid unit, and if I ever take the cells out I'll mark them for the same placement.
Unless someone thinks they would benefit by rotation like tires:thinking:
Regarding the OP's question, the center piece is a metal rod, I'm not sure what he is referring to that wobbles. The other shaped plastic inserts just keep the columns aligned and together. They don't wobble on any of mine, and I doubt they would break since they really don't carry the tension. Make sure that they are in the right places over the battery gaps. If the batteries themselves wobble then you haven't tightened up the holder sufficiently.