Mountain Hardwear - Lamina Eco AF 15F

If you're not into making your own synthetic quilt as suggested above, some sleeping bags come in both a right hand zipper and a left hand zipper and can be joined together at the zippers. Looking now I'm seeing fewer options that do this than before. ☹️ For example the Women's and Men's Mountain Hardwear Bishop Pass bags are left and right zipper respectively, but neither the Kelty Cosmic Down 20 nor the REI Magma bags seem to come in alternating zipper sides. Mountain Hardwear Lamina Eco AF comes in both left and right zips. It's a synthetic bag so it's cheaper but will be a bit heavier and take up more space in your pack than a down bag. But so would a MYOG quilt as described elsewhere.

My Mountain Hardware Ultralamina 15 is 3 lbs and I have used it comfortably below 30. Maybe you can find one used for that price, as bags like mine are 12+ years old now.

If you can find a Mountain Hardware Lamina -9c that's a pretty good option that's often on sale. Big Agnes has some pretty good womens bags. Buying used also a good option to make your money go further unless you have hangups. I would much rather have good used gear than the Coleman / Decathlon garbage people push on this sub.

I use a Mountain Hardwear Lamina Spark bag for this weather when sleeping without a tent. These are relatively light and not too expensive.

For posterity, the sizing is indeed important.  The non goretex versions are "performance mummy cut" which mean these are smaller.  The goretex ones are the ones you bring for expeditions, these are the big bullet proof tankers known as "Expedition mummy cut".   Inside the Expedition Cut, you can gear up with your belay down jacket and stuff, theres more room and then you can get a -18c instead of a -40c for example. The beefier goretex expedition one is interesting if you sleep straight outside in winter, with no tent.  It stop wind and melting snow wont wet the sleeping bag.   For serious winter stuff you can add a VBL in that melting pot of kit to manage humidity and dew point but its not in the main subject. Things to remember: - Performance cut; - Expedition cut. I personally have the blue and red MHW -40c goretex, the -18c, the -18c goretex and the -9c. Each sleeping bag have its particular usage in my activities.

It's really a personal choice. I would suggest you try the MH sleeping bag. It's a free way to test a type of sleeping bag. Worst case, you buy another one later. I personally use down sleeping bags because they are lighter and compress better. But they are more expensive and take a lot longer to dry if it gets wet.