So, would something like this (Pentax Endurance) be an option?
They also make the same design out of "Lightweight 50D Pertex® Shield 2.5-layer fabric".
This is the kind of design that I hadn't found commercially up until just now. This is pretty much what I had envisioned making myself, it's not fully weatherproof clearly, but ideal as a sleeping bag cover without trapping exhaled air inside. Does seem fairly pricey, but on the flip side making one would be fairly time consuming.
This is another contender:
Comes back to the fabric question, water resistant highly breathable (Pertex Endurance), versus waterproof and breathable (eVent or Pertex Shield).
What I'm particualarly unsure of is how colder conditions impact these materials, are some more prone to losing breathability than others?