It probably is reasonable to think that 2 layers of 5 oz CS in combination with synthetic clothing ( which won't compress as much under you) will get you to about 10*F. And especially with a little boost from a vapor barrior and/or a wind proof outer shell. But depending on how warm you sleep, 2 layers of 5 oz by itself might might be cutting it pretty close.
Cannibal and Warbonnetguy should be able to speak directly to this, based on their personal experience. WBG's first torso UQ used FOUR layers of 2.5 oz CS XP. Which should be about the same in total CLO as what you are considering. If memory serves, Cannibal( a very warm sleeper) was able to stay warm with this combo to around zero, while the manufacturer- WBG- was OK at some higher temp, maybe 10*F or 15*F. I have one of these, but I have not tested it below 27*F.
When you first test it for yourself, it might be wise to have a piece of closed cell foam pad for backup.
If you make something for yourself, I highly recommend a differential cut design so that you can pull it snug against your back, eliminating any gap/drafts without having to worry about compressing loft. I think that is a real key to easy UQ success, without having to go through too much of a learning curve. I think some people report being a good bit warmer with regular UQs than others, and I suspect a lot of that difference is success( or lack thereof) with adjusting adjusting the quilt just right. Which is very easy with something like a JRB Mt.Wash, Speer Snugfit( both usable on a HH) or WB Yeti. Or my WB torso length CS UQ. (These last two probably need a zipper mod to be used on an HH, so they can be adjusted once you are in the hammock).
A Speer Pea Pod, OTOH, is one non-dif cut design that has a great advantage, though it would also require a zip mod for use with HH. You also have to be careful not to adjust it too tightly so as to compress loft with the Pea Pod. But, the advantage of a non-dif cut is the addition of clothing or pads and/or space blankets and/or whatever to the bottom, after adjusting the pod a little looser than normally, adding to fill up the space. This makes it easy to take the already very conservatively rated(20*F) Pea Pod to zero or below. Just by using whatever you are not already wearing to bed.
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