Some years back, I made a down mummy bag, and if I do say so myself, it turned out pretty well. I discovered that baffle design is tricky.

I think 1.5" baffles are too tall for a 50-degree quilt. I say that because I have a Western Mountaineering Ultralight, which is rated at 20 degrees, has 5" of overall loft, and has 1.5" baffles (as near as I can measure them.)

When stuffed with enough down to prevent shifting, the down chambers will bulge outward. The tricky part is estimating how much.

On the bag I made (before I learned about this), I wanted 5" of overall loft (2.5" on top). So, I calculated the volume of the down chambers based on a slight bulge, measured the fill power of the down I had, and used 2" baffles. As it turned out, I needed quite a bit more that the calculated amount of down, because the bulge was greater than I thought. The result was a warmer bag, but with about 4 ounces more down that I had figured on. The final result was: total weight, 35oz. with slightly more than 5" of overall loft.

For a 50-degree quilt, though, I would think you could make it sewn-through rather than baffled. Take a look at the Everlite bag from WM, for example:

http://www.westernmountaineering.com/sleeping-bags/extremelite-series/everlite/

An easier and cheaper option would be a synthetic quilt. There wouldn't be much of a weight or bulk penalty in a warm-weather bag like that. Maybe one of Ray Jardine's quilt kits for $130 or so.