As I have bought a new HD bivvy and liner for car camping and my winter basecamping I won't be needing my overbag anymore. I made this a couple of years ago for my USA trip.
It uses a lightweight textured poplin outer fabric [Taslan] and a nylon taffeta lining.
12mm Climasheild insulation. Weighs 1100 grams and has an extra layer of DuPont Thermosheild at the foot and the hood. It is very generously cut and will fit over most winter sleeping bags ( it should fit over any -30 bag ) and because of the size it weighs 1650 grams in the dry bag [ cheap drybag] but if it was Nikwax treated you should not need a bivvy cover and that would save you some weight. My experience with overbags is that they add 8 to 15 C to any bags temperature rating and this would be a really cheap way to boost your light summer bag to well below freezing. A few side seams are coming undone and I would say that the new owner could hand sew these up in a half hour or so, At the asking price I can't be bothered setting up the sewing machine. Cost me ~$150 to make plus quite a few hours of labour.
Asking $90- or make me an offer