I got the idea from seeing some pre-quilted fabric. Instead of paying $12 a yard for that, I decided to pre-quilt some of my own.
These measurements are adjusted just a little bit from what I used. I didn't take into account the bulk the quilting would add. I would have liked a longer flap over the opening, so I've added some length to the measurements below for you.
What You'll Need:
Fabric for Front:
Exterior: 5.75" x 7.25"
Interior: 5.75" x 8"
Batting: 4.75" x 6.5"
Fabric for Back:
Exterior: 7.75" x 12"
Interior: 6.75" x 11"
Batting: 4.75" x 10"
Thread to match
Sandwich your front pieces wrong sides together with quilting in the middle. You can also go ahead and lay out the back pieces in the same way. This picture is actually of the back pieces. When you lay out the front pieces, leave some extra room at the top so that your interior piece isn't perfectly centered (You can see this better in the next photo.) The back pieces need to be centered.
Pin and quilt in place. I just did a simple diamond pattern because I've never quilted before and this pattern is just straight lines. Make sure you sew all the way to the edges. I didn't an regretted it later. Oh, and see how the interior piece of the front sticks out at the top. That's so you can make a seam at the top.
Fold the excess down once. And then again covering the edges. Top stitch in place. (ONLY do this with the front piece of quilting! Do not do it with the back piece.)
Now you can place your front and back pieces wrong sides or interior pieces together. Place the front section near the bottom of the back but leaving enough excess to fold the seams over as before.
Fold the excess from the back piece over once. And again a second time covering the raw edges of BOTH front and back pieces. Pin in place and top stitch around.
Then I took a piece of elastic thread and pulled it through the top making a loop for the button closure. You can see that I had to undo a couple of stitches first. I didn't want the knot showing. After the loop was in place, I top stitched again to close up that little opening.
Add the button on the front and you're done.