Welcome to the forum!
I think it would add too much weight if you just added nylon to cover the cotton (even cutting it down to shape).
Since you are new here you may not be aware of all the great articles in the non-forum part of this website.
Go here to see how to make your own down quilt.http://www.backpacking.net/makegear.html
Scroll down and look on the left hand side of the screen
I've made a synthetic quilt and it wasn't too hard. The hardest part is keeping the silnylon from slipping all over the place. But, if you practice by making some stuff sacks and other things you'll quickly get the hang of it.
You can also get lots of good information over at www.HammockForums.net.
Enjoy and let us hear back from you and oh, take lot's of pictures. We like pictures!