I am having a similar situation going on too. I need an under quilt for my Hennessy Expedition...but under quilts are pretty expensive, and the one I wanted from Molly Mac Gear has been out of stock for close to 2 months (and I recently heard from the owner that the sewers are no longer with them so they are not sure when they are going to be in stock), which bummed me out. At 11.6oz, $135 for a full length UQ good to 40F is pretty god! Most other for that weight and price are 3/4 which I do not want to fuss around with a 3/4.

So I got to thinking...what about using reflexcell? Or a Space Blanket? Sew in some corner loops to attach some kind of cord and walla, UQ. I am not sure how low you can get down too, but worth some testing in the backyard.
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"To me, hammocking is relaxing, laying, swaying. A steady slow morphine drip without the risk of renal failure." - Dale Gribbel