I'm not convinced you need a shock cord on the permiter at all.

If you think about it, as long as it's made so the sides stay
high, you're good. you need enough shock cord at the end to provide some "stretch" so when you get in the hammock the quilt is held snug against your bottom, but isn't pulled off the hammock (it's not structural). I do this with my poncho all the time (integral designs silponcho) which I shockcord at one end and tie sides high to hold insulation against my butt. You really
just need the shockcord at one end to provide some "stretch" to hold it against you when the hammock is occupied.

Have a browse through here to see what I'm talking about:

http://bofh.ucs.ualberta.ca/beck/pictures/Whitehorse0409/

now with an underquilt, you wouldn't want the insulation to be compressed, so what I'd do is the back panel against me that you consider would be kept snug up by a shockcord gather at one end, then you have no issues with an elastic. The trick then is to make the top layer bigger, so that when the back panel is taught against you the "top" (well, facing the bottom when underneath you) is still poofy and not compressed.

Not sure how to accomplish that well, other than perhaps sewing all the shockcord and ties to what will be the "against you side" - filling the hammock with some weight, attaching the panel so it's nice and snug and shockcorded in, then
basting or otherwise attaching bits of the "top" so you know how big to make it so it's still poofy when the against you side is taught. but done right it would kick butt and require no silly
elastics or other goofyness - I know what It *should* do but my sewing skills are legendarily bad.



_________________________
Any fool can be uncomfortable...
My 3 season gear list
Winter list.
Browse my pictures