|click on any photo to enlarge|
I started out with a chain about 15 or so stitches long and then I just continue in the round with single crochet stitches, increasing 3-4 stitches around the ends. The rounded rectangle bottom measures about 6.5 x 3 inches.
When I felt the bottom was the size I wanted I stopped doing the increases. I made two sets of three stitch openings about two rows apart that I used to insert small rings (made from the same cord) that could be used to insert the straps at the end. I tried to take some pictures of what I did. I suppose you could also crochet in d-rings or something similar. I like to try doing things with no metal or plastic findings if I can.
I wanted to make the backpack as versatile as possible. The strap I made is six rows of slip stitch with the same cord. It's about 106 inches long and threads through the front and back of the top and then to the rings at the back in the bottom. You might not need one that long, but I knew I would be knotting it in places and I'm not a petite woman so I wanted to be able to make the straps long enough to be comfortable on my back even with a jacket on. Be sure to do some measurements for yourself. Better to make it too long (you can always put a few knots in it) than not long enough and have to make another. The advantage of making the strap one long strip instead of joining it together is that I can untie it from the bottom of the bag and doubled it can serve as an over the shoulder or crossbody strap.