I have an MEC Emperor Penguin overbag. Relatively light and it adds maybe 8F degrees to my +23F rated down bag. Ray Jardine sells quilt kits. Never seen one in person, but worth looking at. He uses a synthetic insulator. I'd try a quilt before a liner. I've used my down parka like a quilt, but it weighs as much as many bags and cost more than most. It's the one I'm wearing in my photo.
Before making something, I'd look for a used quilt on a site like BPL, here, hammockforums, one of the other UL sites or eBay.


Edited by TomD (03/29/15 10:09 PM)
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