I've been think about making a top bag with two layers of 2.5 over my torso and just one layer over my legs.
I've noticed that when I am pushing the temperature limits with my current bag, I get cold around my torso, so a second layer there would push my temperature limit down.
I'de have the entire length be 80 inches, with the torso double layer 40 inches. The insulation is 60 inches wide. Cut two pieces long enough to wrap around, which would be 46 inches for me. Cut one piece into a 40 inch and a 20 inch wide piece. Then I'de have no waste. There'd be a seam 20 inches from the top but it wouldn't matter because of the second layer.
Maybe something like that would work with a quilt.