I sew cargo pockets onto the kilt with the top edge about 3 inches above the fell (that's from memory and is not a hard-and-fast placement, just a starting point for you to consider). I attach them by opening the top flap and sewing through the very bottom edge of top flap, where it attaches to body of pocket, all the way across, through all layers of the kilt. Takes a zipper foot (so your pressure foot is not at an angle) and a strong machine to get through all those layers! The sides and bottom of pocket just hang free.

There are some pics on my website that might help you visualize this method.

Scroll down to pocket pics. See the photo with pen and cell phone? Stitching line is parallel with top of cell phone, just above finished top edge of pocket, so that only the top flap is held to kilt.