Thanks for the kind words!
The bottom is sewn similar a stuff sack, rolled once and triple stitched. Then I square off the corners, again similar to a stuff sack, to give the bottom its square shape. The long seam against your back is felled (double stitched).
I've read all the bad things about sewing cuben, the stitches pulling out and such. I have to say, with this thicker 1.5oz/sq yd material, it's just not the case. The material actually does very well with bartack stitches. I originally had sewn the top of the straps to the pack with an x-box stitch, but the thread was actually breaking (not the fabric). I think this is because, unlike silnylon, cuben does not stretch at all. I went back and bartacked the entire x-box, and have a super tough connection now.
Yes, the bottom of the straps are sewn into the pocket, with some cuben backing on the inside as well. These too are bartacked. I made another one of these packs for a friend, who prefers a hip belt, so in this case the bottom of the straps are sewn directly onto the hip belts as seen in this photo...
I hope this helps! Good luck on your pack...looking forward to seeing it!