For this project I used the SG rectangular sleeping bag. The starting weight was 28 oz. The website lists the weight as 21 oz. I think this is the weight of the mummy bag.
My goal for this project was to make an inexpensive summer sleeping quilt. The total cost of this project was $33. That was the cost of the bag with shipping. I already had the thread for this so it is not figured into the cost.
This shows the stock sleeping bag opened. This weighs 28 oz.
I wanted the final weight to be under 16 oz. That seemed to be the weight of a lot of the summer sleeping bags on the market. This would also get the weight down to that of most bag liners.
I trimmed 5" off of each side, the zipper and draft tube off of the bottom along with a few inches of material, and the draft tube off of the top. I tapered the bottom corners to save weight. Leaving them in would only increase the size of the foot box.
Note in this step how the bottom black material and the insulation was cut about 1.5" shorter than the top grey material. This was to save weight and inprove the edges. I double rolled the edges over. I made sure that when rolled they overlaped the insulation. This was to keep the insulation from moving inside the quilt.
I sewn the edge seams all the way around. On the top edge I removed the draw string. I folded the nylon from the drawstring inclosure and sewn it to cover where I removed the top draft tube.
I sewn the bottom 24" of the quilt for a foot box. I could have left it open and used a zipper or velcro. Sewing it was the easiest and lighted open.
Below is a picture of the finished quilt. It shows the top and bottom views. The final version is only about 45" wide. I forgot to leave extra wide for the seams. It is also just long enough to come up to my chin. I am 6'4".
The final version weighs in at 15 oz. 1 oz under my goal weight. If I was shorter I think I could get it lighter. It was kind of fun to cut away material and think "I'm not going to carry this piece, or this piece." With my warm bag weighing in at around 34 oz, I could not justify a summer bag unless it was a lot lighter than my regular bag.
After completing this I think it is really doable to make any synethic bag into a quilt. It should be similar for down. I also think it would be really doable to make an underquilt out of a sleeping bag.
This is going to be my summer quilt for my thru this year.