I'm working on a similar bag at the moment.
It's my own pattern, but very simplistic. I'm using a technique that I used for my cell phone pattern found on my blog...
Using size 5 needles, I used SugarnCreme Cotton in Ecru
*k1, slip 1* always slipping last stitch
Work till desired length.
Seperate work as follows
First stitch to one needle, second stitch to another needle, you will find that you've created a 'pocket'.
Flip work inside out.
In order to make my strap without seaming, I put two stitches on scrap yarn, and cast off the rest of the front edge, placing two stitches plus two more from the back side on scrap yarn. I then cast on to my right needle two additional stitches and began k1, p1 for the flap, when I reached the last two stitches of the last row, I casted on two additional stitches, and put the remaining two stitches on the scrap yarn, leaving four 'live' stitches on either side for straps.
Continue working the flap in seed stitch of k1, p1, till desired length, bind off in pattern.
Pickup live stitches on one side of bag and work the k1,p1 on those till desired length and graft to live stitches on opposite side.
Simple cute painless
You could easily incorporate stripes, by adding a contrasting color as you knit the body, and not make the flap at all. The varieties are endless.