Consider putting vest and jackets/parkas on "backwards", with arms through the sleeves/arm openings but zipper(opened) towards the back. This really seems to improve the loft above you to help fill the gaps.

Good idea, but as a side sleeper not much help.

Another thing to check: I have found the down tends to shift towards the ends, surprisingly. I would think it would drift down towards the low point, the butt.

That was it. I checked and found a gap at the center. Observing the Pod from outside it looks like a distinct crease right across the center. Seems that moving about tends to force the down away from the center and towards the ends.

I am with you on the leg area sag, but I don't think I have ever noticed any on the head end. I'll have to watch for that next time. I have to be careful most of the time or my head will really over heat using the pod.

Going to try AHE "Thingies" installed upside down at both ends.