I just made an "underquilt" with Phat's rain poncho idea in mind. Instead of my poncho, I used a nylon ground cloth that is with me all the time anyway....didn't want to tie up my poncho, in case it rained.
Anyway, the nylon sheet is exactly as long as the Hennesey, and 59" wide. It's hemmed all the way around. I sew'd tie loops to all four corners using grosgrain. I folded the sheet in half, and sew'd loops at the folds, so 6 loops in all.
With the thing folded in half, it holds the quilt batting just fine. I put a patch of velcro to hold the ends closed...easier to work with. Shock cord holds it about where my head would be. The other end is right at the hammock bottom opening (i only need my back insulated here in Texas). If I want to insulate the entire hammock, I simply unfold it and un-double the insulation.
I found that prior to getting in, the quilt simply needs to roughly form with the bottom of the hammock. With my weight inside, it's tight enough to stay in place, but not crush the insulation, or billow in the wind. It can still be used as a ground sheet or gear tarp if I'm on the ground.
Seems warmer than a foam pad (i can shove the pad in the quilt too!) and a heck of a lot more comfortable!
Basically its a big pocket with tie out loops. 1/8" shock cord does the trick on the tie loops.
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paul, texas KD5IVP