Another straightforward puzzle with a frustrating endgame...
"Symbol" is rather ambiguous if what is required is a letter. The obvious letter would link nicely with the title of the poem, but for some reason the BRB does not have it with this meaning although it is widely used. (The same goes for its opposite partner.) Or maybe it's an abbreviation of the last word in the poem...
I'd go for an outline of the letter in the blank cells – TWO dimensional rather than ONE, as loge mentioned in post 8! This is a more satisfactory interpretation of "drawing...in the... blank cells" than deploying the highlighter pen to fill in what is already obvious by virtue of the cells being blank.
Anyone have a better idea?