Success!

• lining was part of an old towel. This works well for the current purpose, because it also stops the keys jangling
• I didn't do the calculation correctly. I did a 'about this big', concluded that the width of the piece of towel minus selvedge was 'about right' for the length, and then just cut a strip of the width plus seams I wanted - I laid the two pieces of fabric on top of each other, and cut together with the rotary cutter.
• for this size pouch, I went with 1/8" seams, but as I'd cut the fabric for twice that, I ended up with a slightly larger pouch
• I cut the flap just slightly narrower at the top; I should have done this on the cutting mat; I cut one side and then used the piece as the template for the other side, and it is decidedly wonky.
• with the thickness of the towel, the top stitching wasn't a great idea, so I whipped stitched it closed. I do enough whip stitch that it is mostly invisible.
• I used a piece of Ofray ribbon, which I happened to have on hand. I think it is 2cm wide. I pinned everything for the four layers, then unpinned to add this carefully with wrong side to right side on what was going to be the front. Then I pinned again
• using quilting cotton and terry towel together was the kind of fun that is solved with lots of pins. They don't want to stay together At All.
• sewing the first seam, I sewed 1/4 on either side. I probably should have sewed less, because it was a bit of a struggle to turn right side out.
• Always check thoroughly for pins before turning right side out - this was how I paid the blood price for the piece.