Re: flex 180 insulation
The wide downmat is comfortable, warm and is locked in by the center fabric ridge of the Flex. You have a little leeway in positioning it for a diagonal lie. I had it when I bought my nx250. It slid around a lot in that hammock. Can't compare the downmat to an underquilt since I've never owned one. The downmat seems to be easier to "rig". Also can't speak for how a 20 inch downmat would work.