I'm through dancing around my underquilt design; I'm just gonna go for it, and see how this turns out.
I've ordered a pound of down from Wilderness Logics, and a few yards each of Impetus 1.0 and 1.1 from Backwoodsdaydreamer. I've previously been fretting over material choices, but I've come to the conclusion they'll all work equally well for my purposes. I finally realized Impetus and Momentum weigh about the same, cost exactly the same, are made of the same polymer, do the same things, and have practically the same name . But Impetus comes in green, and that works better for the quasi-stealth look I'm going for
After a couple tailoring iterations of my muslim mockup, I've nailed down the liner surface of the quilt, and now I need to figure out how to offset the outer face so as to not compress the loft. Since my loft requirement is in the 3.5-4.0" range, the cut differential on the inner/outer surfaces will be pronounced. My "guesstimate" will be the fabric width will be 30% higher on the outer than the inner (this also includes the extra ripples from the baffles). Unfortunately, the curvature of my quilt changes a lot over its length, so calculating the differential will be very difficult without a ton of measurements (I'll try, though).
My baffle arrangement will be somewhat unique; a hybrid of interrupted Karo-Step style, and the typical longitudinal method. There will be several longitudinal baffles that are very long, and have a slight overlap where they meet (but which is not sewn). Between them will be the typical grid of short Karo baffles with gaps at the corners. When stretched out on the hammock, the long baffles will be "taught" enough that the gaps between them will close and prevent down shifting towards the middle. When unloaded, the baffle gaps may open and allow down to be shifted to where it is needed.
The underquilt will resemble a mummy bag with the top portion removed, but still covering the feet and corners of the shoulders. I'm unsure exactly how the baffling will work in these highly-contoured areas, but they will be baffled off from the rest of the quilt and overstuffed a bit to ensure I get the loft I need at these extremities. It will attach to the hammock floor along the spine with Velcro or button-snaps to eliminate gaps, and be pulled taught by shock-cord in channels along the sides. Because it wraps over my feet and shoulders, there will be no need to seal the ends with draft tubes. I will make a separable hood to insulate my head/ears.
I've got a fair bit of work ahead of me, but hopefully having the materials staring at me will light the fire to get this done. I've found it's really hard to be productive while lying in a hammock , which is why it's taken so long to get the quilt design/fitting nailed down. I lay back to take notes on where fabric needs to be added/removed/adjusted, and just sorta forget what I was supposed to be doing...