While surfing some other backpacking light forums I came across the following item that I think would make a good candidate for either an under quilt or top quilt. Here are the specs.
4" loft (for the total bag i.e. both layers.)
90/10 down. 90% down 10% feathers reading the fine print on the fill power link on that site gives more details on that.
2 lbs 15 oz. 85" long 30" wide when flat (60" total circumference)
$99 plus shipping