Do not store a feathered insulation system in a plastic bag.
The feathers need to breath and loft for optimal long term storage.
Repeat treat and store the under as a down sleeping bag. Large cotton/linen
pillow type case would be best
Snake skins, some of the ladies on womenhiker lists who sew have tried to
made them. Many have found It's best to take some discarded fabric and make
several 2 inch wide circles to determine what diameter is best for this as
your tying to compress the feathered underquilt with the hammock as if they
are one piece. That way your have a good idea which diameter you want the
finished snake skins. I like the loop idea to help pull the two halved
together in theory might not work well with the bulk of the underquilt. I'd
make then to over lap a bit with some sort of closure possibly Velcro as
your tying to contain
a feathered stuffed item and I'd make it with a breathable fabric for the
down to compress efficiently.
Just some thoughts on your questions. Trial and error will tell you what you
need the most though.