Another option is to use a low cost VBL (Vapor barrier liner) inside your bag. VBLs are often controversial--you either love them or hate them. I frequently use one to extend my 30F Marmot Hydrogen into the low 20's and sometimes teens--by also adding a fleece neck gaiter and 2 fleece caps. With a sleeping bag VBL you can also 'adjust it'--by rolling it down so only your legs are covered...or use all of it for maximum warmth. Here's a cheap one I use (polyethylene not mylar) that adds only 3.8oz:

https://www.amazon.com/gp/product/B000WX...pf_rd_i=desktop

You can even carry two of these--one as a ground cloth and the other as a VBL.