Just finished my IX Underquilt today using rjcress's excellent instructions at his gear review site. Ordered all the materials from DIY Gear Supply and although it took 3 days of off and on work and fiddling around trying to decide exactly what I wanted (or needed), I'm quite pleased with it. Tonight is going to dip into the 30's, so I will be out getting the hang of using an underquilt for the first time. I'm moving up from pads and big sleeping bags.
I had an old 40 degree Wiggy's sleeping bag that finally wore out after 15 years, so I took it apart to reuse the Lamilite insulation in the draft tubes I put in the underquilt. They're 3 inch tubes all the way around. Snugs up real nice with no gaps that I can see or feel. I used pleats instead of darts (6 inches long) on the insultex and got a good bowl shape. This was my first "big" project - lots of mistakes but a lot learned about thread injection.
Now the family can have the living room back. For awhile it looked like a fabric store blew up.
Next project starting tomorrow is a hammock sock! Was told to take it out to the shed this time.