American Sign Language:  "CL:L" (classifier "L")

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RECTANGLE, check, envelope, card, brick:

(On YouTube at:  https://youtu.be/QZGTQ4FBOfQ )

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CL:L
(Classifer "L")

SQUARE or RECTANGLE:
(Can be modified to show small square flat things such as an envelope or card depending on the size and shape of the object you want to depict)
 

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Animation:

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