I have a proposal I made just now for the sign:
In Celebration of Derpy Day
Courtesy of Operation: Indiscriminate Friendship
I would take it to a local printing shop and haveit printed on A1 or even A0 sized poster board and set it on an easel or something like that, but you can't use that image due to its low resolution. Not sure how much that would cost, though. Email or PM me for the print-quality file if you're interested.
Also: Just registered today just so I could post this.