I just came across a device made for / by Big Agnes they call a "Wedgie."
Comes in two lengths for two typical sleeping bag (sb) zipper lengths.

Simple, but too damned expensive: A 50"-60" V shaped down-filled draft tube with zipper halves that mate to the zipper halves on the sb .

I just mistakenly bought a fine 20 degree bag......for some narrower people. I intend to use it as top quilt. 59" at shoulders should be plenty / too much width in a hammock. But for other uses of the bag, this expander might be the ticket to make a trade unnecessary.

Any experience here? Are sb zipper's standard enough so one or 2 products would be compatible across bag mfgs?

I wonder if there might be enough demand for one of the cottage industries to make a product with same function. Or knowledge to tell me not to bother even trying one.