The Hudson River quilt from Jacks'r'Better can be used specifically but not limited to an underquilt. It has a omni tape section and drawstring to use as a partial mummy quilt. I have overstuffed this quilt with 2oz of Speer's 900fp down and everything lofts up to well over 3 inches.
As a top bag, this will easily reach the low 20's temps.
the seam along one side was opened very carefully, and then sewn back shut with Gutterman (black) thread. It has only seen 4-5 uses, mainly as an underquilt.
asking $175 shipped
comes with stuff sack
has the foot-end drawcord modification
(the custom overfill is worth at least $40 considering time + materials )