I just purchased some 16x13 sealable sleaves (actually 16.25x13.44) but I'm not aware of any 16x13 boxes so I'd be in the same boat. 15x13 also allows for 6 boards per 32x40 board with a bit more waste though but there is the ability to remove a little from each edge which quite often is the case because of rough handling on the bigger boards, at least in experience.
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