I've got the material and the down for a new UQ to replace my rather heavy 45F (try 50F) mummy bag. I'm still debating on the design though.
My UQ has about 2.5" of loft on average, and uses polyester mesh for 'wall' type baffles every 4.5" or so. The result is very good, and it seems extremely warm and durable, but it was a real pain in the hind parts to assemble the thing.
Looking at a lot of the 40-50F down bags like the Lafuma 600, it seems most don't use real baffles, and instead just sew the inner and outer liners together to form tubes with stitching sewn all the way through the bag. This would be a simpler construction method, but it seems like it would also pretty significantly de-rate the bag, yes?
Are there simpler baffle designs that don't radically increase weight, but don't require so much pinning and sewing (two seams the length of the bag per baffle, 15ish baffles).
Also, how much loft should be a rough starting point to get to 40F or so comfortably? I'm a pretty warm sleeper. The 2.5" on my UQ was almost too warm at 55F last week, so I'm optimistic about its performance on the low end, and I'm leaning toward sticking with those dimensions.