So, Plan B:
First, I cut the pocket from the curved between-the-strap pieces left over from cutting the body because they are a nice shape and why not use up those scraps. I used two pocket pieces and stitched around sides/bottom RST, turned and pressed, then added a teeny pleat in the center top, and trimmed the top edge with a bias strip to match bag lining:
Then I assembled the outside (underlined with flannel) and lining bags. Here they are, ready to go, including a button loop on the lining:
At this point I left and did some grocery shopping, thinking I was done with this for the day.
Still couldn't leave it alone, so got back to work on it after errands. Next up was the tricky bit. I needed to make a third "body" from the too-stiff-too-heavy interfacing I planned to use. I'd cut it to size, but decided to trim the seam allowance off to minimize bulk.
I cut another piece of even stiffer interfacing to fit the bottom of the bag, and slip-stitched it in place to the SA on the ends. (Here it is, before stitching down. Above it is one side of the bag body interfacing; the bottom, instead of being seamed, will slip below the heavy interfacing piece. Had another pic that showed that better, but it was unusably out of focus).
I used the triple-zig-zag stitch to sew the sides of the interfacing together, just abutting, so no overlap or seam allowance. That photo didn't come out either; I seem to be an even worse photographer than usual today, sorry about that.
At this point I've got: exterior bag, lining bag, interfacing bag. After a tremendous amount of fiddling (the flannel-interfacing combo was ultra non-stick, didn't make it easy), pinning, and a few curses, I got the lining bag inserted into the exterior and pinned in place.
So far so good, and hey, it's got "standup-ability":
Now I just need to trim all those layers around the body-strap edges so they're even, and sew bias trim around the openings to finish it off (and hold it all together).
I suspect the bias trim might be a bit tricky, too (am still having doubts about my machine's willingness to handle all those layers) plus I haven't cut the bias strips yet. This time I really am done for the day.
Wish me luck with the finishing bit.
Hope to get to that tomorrow.