I put together a Top Quilt with 1 layer of 3oz Climashield from OWF, seen here in this thread. I'd bought 2yds of the insulation (it's 102" wide), so that I would have enough for an Underquilt to match it.
The building method is essentially the same as the top quilt, check the thread for that info, or my website blog for a writeup on it.
I made it large to reduce fiddle factor (well, truthfully, just because I had enough climashield left to do it big). 72" long, 46" wide. It has 1.5" Gross grain folded over for edging on all four sides to run the suspension and cinching lines through.
5oz (Insultex cover)
packs down very small, little larger than a nalgene.
your guess is as good as mine, but going by this site, should be safe to 40/45.
Also - I built an Insultex cover for it that should boost the temp rating a good bit. No actual field results yet.