Finger over the air hole is a bit of a myth. Even if you have the lower hole covered, the Chimney cap is still open to allow equalization.
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it's interesting you mention this. The kayfun lite documentation, does mention 2 ways of filling. The 1st one being the top fill method, so it seems the manufacturer isn't advising against it. Furthermore, the top fill is a straight forward process, there is no mention of putting finger over air hole etc.