I was reading other threads about using a US Army poncho liner as an underquilt (UQ), so I thought I'd give it a try. I went down to the Army/Navy surplus store and picked up two brand-new poncho liners for $15 a piece (your mileage may vary). At first I was going to do a full-length UQ, but after reading about 2/3-size UQ and looking at how large the army poncho liners are, I considered making a smaller UQ that would also have twice the insulation!
1 - US Army Poncho Liner, standard size
20 ft - 1/8 in (3.175 mm) shock cord, black
Lighter, or a few matches
Small gauge wire for threading the shock cord
This project is actually very easy. First, lay the poncho liner out so you see three sets of tie-outs on the left, three sets of tie-outs on the right, and a tie-out set on the top and bottom. Next, fold the poncho liner in half, like a hamburger bun, so you end up with the fold on your left and the open side on your right.
Going clockwise, you should now have one set of tie-outs on the top-left, two on the top-right, two on the right, two on the bottom-right, and one set on the bottom-left (see attached illustration).
The tie-outs on the right side can be tied together to keep the poncho folded and create the two layers of insulation.
Next, cut the shock cord into four (4) equal lengths of 5 ft (1.5 m) each.
The next step is to thread the shock cord into the edging on the poncho liner. Each length of shock cord will thread through the edging on the poncho liner and pull in opposite directions to gather the ends of the poncho. Using the scissors, cut small slits on the left and rights sides in the edging. To prevent the fabric from unravelling, optionally melt the nylon ribbon with a lit match or lighter, being careful not to ignite or melt the entire poncho liner.
Next, thread the shock cord through the holes and the fabric channels on the poncho liner. One shock cord threads through the right, another cord through the left. Tie off one end of the shock cord to one of the tie-outs on the poncho liner. The free ends of the shock cords will attach to each other and the hammock.
Pull the shock cord to gather the fabric of the poncho liner. Tie off the shock cord to the hammock, adjusting the length of the shock cord for a custom fit.
Optionally, you can use cord locks to tie off the free ends of the shock cords.