I have no experience with a regular bed quilt. I don't no what temp it would be rated to and would really hate to give you bad advice. I made a down underquilt from a Kmart Cannon pillow that had 44 oz of 500 fill down and it came out great. The pillow was $16 and materials total was about $60 with the pillow. If you can sew, then posibly think about this route.
Here's a guide to make a poncho liner UQ http://www.hammockforums.net/forum/s...ad.php?t=18539
Possibly you can use one of the quilts doubled over in between the poncho liner. I wish I could help you more, but I'm not sure about the down comforter. I'm pretty sure something could be done and hopefully someone will chip in soon.
I'm in Indiana and feel your pain about winter and a low budget.