Because you are thinking "special purpose", consider custom design for your hammock.
Draft stoppers and collars may be inefficient use of materials. Traditionally, on SBs, they fill and insulate a space along zippers, or are smaller-circumference collars, applied full length. In contrast, when used at the ends of hammock UQs they are expected to be scrunched, stuffed like backing for caulk to fill furrows.
I'd suggest instead a short trapezoidal skirt of low permeability fabric sewn to the end and up the side seams of the UQ, just long enough so the inner hem overlaps and encloses the last few inches of the UQ. That hem has a channel and is threaded with a light shock cord and a cord lock. Total weight for both skirts, head and foot ends? An ounce?
The full length UQ gets mounted and the skirt hem gets pulled just snug against the loft. The seal here,of a few inches of skirt overlapping a few inches of UQ is excellent, with the minimal compliance of the U. Air passing up and down the furrows beneath the UQ, such as when feet shift and those furrows re-form inside the hammock, cannot escape the skirt because the skirt is sewn on three sides, and cinched on the fourth. Airr cannot get in from the outside either.