Did you scuff the ends of the cells a little with sandpaper or a dremel with a soft sanding whell before you soldered them? That usually helps with mine....
Try padding the bottom of the battery brace too. Might help by keeping the cells a little tighter with the padding underneath as well. I use foam sheets in 2mm from craft shops and stick them in with thin double sided tape. (you can colour-coordinate with hte foam as well
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