To save some extra weight, try this.
If your width at the top of the sewn foot-box is say 40", then use this width all the to the top.
The rest of the quilt width on the bottom flaps do not need as much down and the one seam on each side (going vertically) can be sewn through.
If you have a quilt that is 2 1/2" thick on the top, and the sides are 1 1/2" you will save at least 3/4 of an ounce.
Having baffles on the 40" of thickness also keeps the loft on top as it can not shift down.