I finished my underquilt using the plans in this thread. I used 3 layers of Insultex. It was my first thread injector project (not counting hemming up a couple pairs of pants) using my Viking Husqvarna sewing machine that I purchased from a garage sale. It took about 4 hours to sew up. The hardest part was threading the shock cord.
Here's the finished product:
I used camo ripstop on the top and bottom and ironed the folded over Gross Grain to aid in stitching them to the underquilt.
While I was at it, I made a little stuff sack and it came out pretty nice.
The overall weight in the sack is 18.45 ounces which is quite acceptable for me.
Thanks to SCLittlefield for posting the plans and providing the encouragement to take on this project.