For this particular application, I would use nylon monofilament (or polyester monofilament) in the top, the bottom, or both. The one I like and have used is YLI nylon monofilament:
http://www.amazon.com/YLI-Corporatio...dp/B00A2GSI3I/
There are some tricks to using it because it stretches:
(1) Lower your top tension. On my machine, I move it from 5 to 3.
(2) If you use it in the bobbin, be sure to wind the bobbin slowly to minimize stretching. For a plastic bobbin, fill only half full. For a metal bobbin, you can fill 3/4ths full. Because of the stretch, overfilling a bobbin can cause the bobbin to distort and give you lots of sewing problems.
(3) Use a thread net over the cone. It helps a lot to use a thread stand to feed the thread to your machine. (I really like Superior Threads' latest thread stand.)
If you do a search for "monofilament thread" on the QB, you should find other comments about it. Some people prefer polyester monofilament over nylon. Superior Threads makes a good polyester monofilament. In my experience, polyester monofilament is not quite as invisible as nylon monofilament; however, it is still very invisible.
Edit: Harriet Hargrave is the quilter who introduced nylon monofilament for machine quilting to the world. Her book on "Heirloom Machine Quilting" provides many tips for successfully using this thread.