Hopefully the various "fan displeasure outlets" can coordinate a little so a singular message can have the largest impact.
If this continues to build, a coordinated effort with each successive game.
Keep the message clean enough to appeal to the masses. For example a "blackout" where fans attending the game all wear black as a reminder to Clark of what will happen to attendance if this team continues its course.
I also think its important to keep a message of general player support. We still (mostly) love the guys out there playing their hearts out. I get the need to single Cassel out, but ultimately I want the players to realize we support them and this is directed at the FO. I think that will be more likely to appeal to the masses.
Courage is not the absence of fear but rather the judgment that something is more important than fear. The brave may not live forever but the cautious do not live at all.