Our winter duvet is organic bamboo fabric and fill. Very comfortable and toasty and has not gotten mashed down over the years. Do not remember where it came from now, but this would be fairly similar:

http://www.cozyearth.com/bamboo-comforters/bamboo-comforter.html

I got a very nice canadian goose down from costco with 750 fill. Its amazing

The best kind of duvet insert is cotton. At Live
Good, we have 100% organic cotton duvets, which a nice for any bed.