I am assuming that, with the use of the words square and miter gauge, you want to make one end of the board exactly 90 degrees too one edge of the board.
As already suggested a cross cut sled is your best bet but you could use a router and a straight edge, though I would be more inclined to go with a circular saw and a straight edge. Then there is saber and hand saws.
It must be one hell of a board. If it is that wide I can only make the further assumption it is also long and weighty. If that is the case a miter gauge wouldn't serve you well anyway. It would be to hard fighting the resistance of the board on the table to keep the board from cocking away from the face of the miter gauge.
If that isn't the case and you are dealing with a short squat board you can just rip it against the fence. It's a fairly safe operation if the board isn't too much longer then it is wide. Approach the process carefully, if the board is too much longer then it is wide it is a prime set up for a nasty kickback.
It's a question of leverage and bearing surface against the fence. Too long and/or to little bearing surface gives the blade a high leverage advantage and you'll be pulling the board out of the back wall, and that is only if you are lucky.