If it were me, I'd draw my box/rectangle/shape in Illustrator, and use one of their pencil stroke brushes to give it that (more) natural look. Save it out as an AI file and import it into After Effects. Then you can just scale it to whatever you need it to be, animated some masks if you need to do a write-on effect (or better yet, before you do any transformations to it, copy the path from Illustrator, paste it onto the layer in AE as a mask, set the mask property to None, then add a Stroke effect, set the Paint Style to Reveal Original Image, then play with the brush size and keyframe the Start and End properties to animated the write on for you.
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