Placement of the mast box will be in the gap between the stringers in front of the forward bulkhead. This will make a nice tight fit for the box.
Here is how it is supposed to look when finished.
Well when I went to cut the opening in the deck for the mast, I was off center by nearly a 1/2″, so I had make some modifications. To make a long story short, I took out the side wall in the direction I needed to shift the box for it to be center, and then filled in the opposite side with the appropriate thickness of plywood.