Not cheap.. but the Montbell Super Stretch is by far my favorite small bag. I use when packrafting and size is so important.
Have you ever heard of Outdoor Vitals? They have this 20 degree underquilt thing.
https://outdoorvitals.com/product/outdoor-vitals-aerie-20f-down-underquilt-sleeping-bag/
It can function as an under quilt, a sleeping bag or just a big blanket.
It only costs $160.
I am going to get a couple to try out for the wife and I for some van camping, backpacking and car camping.
That looks cool, but all that verbiage on their page and not a single mention that I can find of the dimensions.
Outdoor Vitals, it's great that you're, "pushing the boundaries of quality, price and innovation via our direct-to-consumer business model and amazing company culture", but how long and wide is the thing you're trying to sell?