Take a look at this post I did a while back.http://www.hammockforums.net/forum/s...07&postcount=8
If your bag is down, that will change how well this approach works.
Like was mentioned earlier in your thread, two stitch lines along the area you want to cut will help seal in the down. Trying to do that on your machine may prove awkward to say the least. Might need to tackle this project with a needle and handstitching.