I did something simlar to your proposed design. I don't quite understand the extra zippers? But anyways.. a bag can work, but from experience it does /can get a little cramped and does not allow for sprawling, especially in the knee area. Supporting the ends with a shockcord suspension helps. Synthetic bags will not pack down small.
shug did a video recently on a bag/pod set up, give that a look.
Here's some pics.
GargSB final 006.JPG
GargSB final 008.JPG