Originally Posted by AndiR

Wool would have been my first choice too, but since you can't use wool, Polydown is my second choice. What will also emphasize your feathers is to do a tight fill around them. That makes them 'pop' more. Quilt the feather in a heavier thread that maybe contrasts with your fabric. Then do the fill in a thinner matching thread.

The quilt below has none of these suggestions, except for the tight fill ;-). This customer prefers cotton batting, but if I'd used wool or Polydown the feathers would have 'puffed up' a bit more.