Maybe the side of the quilt that faces the bottom of the hammock needs to be smaller. I've never made one of these, but it seems like you would want the side that faces the hammock to be a bit smaller than the side facing the ground... that way you get tension against the bottom of the hammock without compressing your down.

Also, are you talking about sag while you're looking at it or while you're in it? I think Brandon warns that there will be sag in the Yeti if you're just standing there looking at it, but it goes away when you get in the hammock.