Here is a link to someone that posted directions on how to make a version of a down quilt. It could be used to help make an underquilt. I think it is similar to what thru-hiker uses for a kit, or what they based their kit off of. I forgot about this one, dino posted it over at WB. Here is the thread where she did that people are talking about making a quilt. Do some other searching there. I remember reading other threads at some time.