so after reading a "fanny" load of info on uq's . I made a full length under quilt out of my 20° alpine design mummy sleeping bag.
It came out great. I basically made my mummy down sleeping bag into a 48x60" rectangle blanket with shock cord pull out on all sides.
It had a middle seam that I measured off of to get the shape . Hemmed the sides up with bias tape. Ran the shock cord in that. Put the extra down from the hood and sides, I cut down in the middle six channels.
it took a person that is good at sewing with my direction 3 hours.