Simple, but effective. I used an Aussie poncho liner for this test, but it should work with a surplus US one as well. I'll test it out on my US ones and report back.
It would be a simple thing in colder climes to use two poncho liners tied together one on top of the other as a UQ.
I'll post part 2 in a day or two which shows how to secure that gap in the insulation around the foot end of the hammock - I think you'll enjoy it - plus photos and maybe a little video of how the thing works.