Been there, done ... something like that. I like your first response of putting pleats in the top layer. Measure that one as carefully as you can if it seems the right shape. That will give you a ball-park figure for lengthwise adjustment as well as widthwise. For my insulated hammocks and similar shaped underquilts I can get a really accurate layer that matches the shape of an occupied hammock using my fairly tedious method of mapping the surface with triangles, measuring all edges, and making patterns from which to sew, etc. However I resort to trial and error when making the second (bottom) layer that hangs evenly below it. I still use the triangle patterns from the top part, but space them a bit farther apart to get a larger curved panel. I also vary the additional lengthwise spacing depending on the curvature of the hammock - smaller radius under the butt. Then, leave extra fabric around the edges, pin it up and test. If you get close to the right size and shape by calculating and measuring, you can really get an even layer of insulation by adjusting the edges. The payback is wonderful - less insulation needed, no gaps, no compressed insulation, and a much warmer UQ.
(Now to go re-read hangnout's posts. We're all still learning.)
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