For stopper knots, a butterfly or alpine loop was my first thought, but when you tie a butterfly in the middle of a span it  puts a full twist in the line. Not optimal. And here they are pulled up short into the block. If the boom were held up off the gallows by the lazy jacks I don't think it  would swing back and forth at all, and the sheet wouldn't have to be drawn tight. The extra slack you see is in the parts of  the sheet no longer able to take any tension because of the stopper knots: 89