I would suggest a 3-rail block with the two outside rails being dark (solid, tone-on-tone, print) and the inside rail being light (white, off-white, tone-on-tone, pastel color). The inside rail would be for the signature. You could alternate the 3-rail blocks with solid blocks so the center rail would seem to float on the surface of the quilt. You would want to have the quilt at the Festival so that people could buy and sign blocks, so you could make and assemble the top ahead of time, and have it at a table so that people could sign it between songs, at intermission, etc.