Funny timing. I just did this over the last couple days while home sick (was supposed to be backpacking :(
I took a Kelty Light Year 25 (31 ounces before), removed the hood, removed the zipper, and essentially turned it into a pseudo-Arc type quilt. It has a knee-high footbox. I had a “bag coupler” thing that had 4 ounces of down in it. Took the down from the hood (about 1.5 ounces) plus the “coupler” down and overstuffed the baffles.
Pretty damn pleased with the results. Loft before was barely 2″ mid-baffle, now it’s 2+” at the baffle valleys and 3+” mid-baffle. Whole thing is 31 ounces now, and ~22 ounces of that is down. Not bad for costing me nothing.
I’d echo whoever said that it’s not as bad to work with as anticipated. I doubt I lost more than a small fraction of an ounce and I didn’t climb in a tent or anything like that. Shake all the down towards center on the side you’re opening up and it prevents loss.
I can give more details if you want. It went well and I’m no expert with a machine.
The bag was good to maybe 35/40 degrees before despite it’s 25 rating. I’m guessing it’s good to 20 now and it’s very room and comfortable.
-Curt