I came up with a design that is partially inspired by Melissa's arm band tutorial and partially inspired by a product called FlipBelt.
Fabric: I used a poly/lycra blend, but it would work with most knits. The recovery of the fabric is important though. If the fabric doesn't have good recovery, it just won't stay up on your arm.
Elastic: My first version didn't have the wide elastic, it worked fine but it is floppy, so I have to put the phone in the armband before I put the armband on. The elastic gives a little structure, so I can put the phone in my armband while it's on my arm. I used 2 lengths of 2" elastic zig-zagged together to give me elastic almost as wide as my phone.
What I did:
First I wrapped some elastic around my arm, and gave myself a little extra for insurance. This came out to be about 14", so I cut 2 rectangles of fabric, 8.5" x 16".
Putting the fabric right sides together, I chalked a line down the middle of the fabric, and halfway point of the line. Then I laid down my phone, and made a little chalk mark just a tiny bit smaller than the phone.
Then I sewed down the middle chalk line, but stopping at the marks, so there's a gap in the middle. Now the elastic is going to get sandwiched in.
I put the elastic down on the fabric, folded the one of the top layers down, and basted it to one of the bottom layers, so the elastic was encased. Then I basted the other two long edges together, and finally all 4 layers together. So now I have a tube of fabric, with a gap in the middle.
I turned out the tube, so the seam was on the inside
And then I folded the tube in on itself, so that all the remaining raw edges are together.
Then I stitched these raw edges together, making sure to catch the elastic in the seam.
And finally used the gap in the middle to turn it out to the right side.
I had to try it on a few times to get the size right. I ended up cutting off about 3" of fabric to get it nice and snug. I had to really trim down my seam allowances too, because the elastic is so bulky.
And that's it! The phone sticks out a tiny bit, but it's totally secure and doesn't move around at all. I also put my key in there, and although the pocket goes all the way around the arm, I have no trouble fishing it out.
Hope that was useful to someone. Happy sewing everyone!