So made my own 5oz full length mylar plastic underquilts. The camo is made from a cut open 16$ Titan Emergency sleeping bag (they have 3 colors) and the orange is from 7$ SOL 2-person emergency blanket. Both of these are made from a tear resistant and quieter plastic material instead of the traditional noisy foil that once rips, is practically destroyed.
The corners are folded about 3 inches and taped with fiber impregnated tape, then four sides are folded 2 inches and taped with gap for shock cord running though and 4 cord locks for the cinching ends. The photos are showing 1/8th shock cord, however, 1/16th was sufficient.

I found these to be very comfortable, waterproof and wind proof. And most importantly 5oz vs the 16-24oz under quilts I had used. They can also double as a rain protection when not in use on the hammock. The SOL ended up a couple inches longer and you won't have to cut it open but the orange is not my thing. These only take a few minutes to make and require no sewing. I also find that connecting any underquilt to your ridgeline as shown instead of the ends creates a better seal. I fold these up and place in a zip lock bag for nice flat storage.

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