I've got a Big Agnes bag (insulated on the top only with a sleeve for a sleeping pad), that has a down-filled collar at neck height which hangs down from the top half of the bag. It reminds me of a the carrying yoke on a canoe - narrow (maybe 2" wide) in the middle and wider (4-5") on each side so you neck fits into the narrow part, so your neck and shoulders are all sealed to the bag to prevent a bulk flow of warm air.
I was surprised how effective it is and, per ounce, it has to be one of the most effective feature of the sleeping bag.
I greatly preferred it to the draw-string collars on other bags - it didn't constrain me, I could roll over just as easily, and it would settle back into my neck whether I was on my side (usually) or front or back.
It would be a pretty easy retrofit - buy a down jacket from a thrift store, use the down from it and the inner nylon taffeta as a fabric. One long seam might already be sewed for you and it doesn't take much force at all, so any old stitching job would hold it in place.