@EthanAdams Check out the categories of bivvy sacks and, specially, bag liners.  Most current  bivvy sacks are glorified one person tents, which is not what you need, but simply a very moderately insulated bag.  There are lots of options out there, especially when combined with varying weights of clothing, etc.

I wish I had your problem.  You could always try backpacking in colder regions..... 

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