Matthew- The part that holds the pad is 6.5oz lycra. It stretches to allow either 1 piece of foam for a frame, or even a full pad. It was the lightest 4 way stretch fabric OWF Inc had. It feels great, but may not be optimal for the sole purpose of lining the back. Thru-Hiker sells 3D mesh, which is what most companies use for stuff like that. For the shoulder straps, I used pack cloth the whole way around. I just like the feel better than the 3D mesh personally- less grippy, slightly cooler. They will get sweaty, but wouldn't absorb the volume of water 3D mesh would. It's all personal preference, but I think most would opt for the 3D mesh.
Tim- The pack is built similar to the Prophet. It could have been 1 back piece, but I wanted to put the shoulder straps in a seam instead of sewing it to the body, and I also wanted to put the pad straps in seams as well, so it ended up being 3 pieces. I like the curved bottom because I feel like it reduces that saggy feeling i've gotten from square bottom packs which pulls back on the shoulders. Its a PITA for construction though, so next time I may the bottom piece a trapezoid instead of a half moon. I do support the use of an acute angle at the bottom of the pack either way. I did use heavier materials than usual. I figured if I can make it 2000+ miles (fingers crossed), my pack should be able to as well. I noticed GG put up a notice that one of their packs would only be suitable for 50 trail days or so and didn't want to be SOL so far from my sewing machine, or heaven forbid buy a retail pack. Also, oxford and pack cloth are dirt cheap and durable. I was hoping to find a lighter spandex, but the 6.5 fit the bill well. I wanted to use lycra to keep the pack streamlined and small, reduce snags, and have highly adjustable volume. It stretches a lot more than I expected which is great. It's difficult to work with though, getting it stretched and sized appropriately.
Here's a picture of it with 11 rolls of TP in the pocket (#12 was in use). I could hire a thru-hiker to sherpa my stuff with 11 rolls of TP. After that is the curve on the bottom of the pack. With at least 3 of my other homemade packs, i had a constant sensation of the weight being transferred to the bottom of the pack away from my back. The curve forces the weight to go against my back, which is nice whether or not I am using the hipbelt