Its not much, but you know how these thing go. I could just leave it, but then it would be boring 
It would allow you to go 22S in the same space 
Looking at all designs i can safely, without jeopardizing physical integrity make modules as follows:
18650 @ 20.5mm vs 22mm
21700 @ 23.5mm vs 26mm
26650 @ 28.5mm
only 2.5mm thicker than the cell itself. Also ~1-1.5mm shorter
Big question mark is if i could reuse my tabs. Making it thinner offsets bolt hole. At the moment 18650 bolt hole is 11mm from the side and same from the top/bottom. This allows to use same series busbar either way. Have designed the holes oval but i am not sure yet if it will be OK.
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