A wider i-cord should do the trick... try an 8 or 10-stitch icord. You'll have to do some swatching, but that's to be expected.
As for the bag, you can mostly follow the directions for the crochet... let me see...
Using black yarn, Ch 2 (counts as first st), 7 hdc in magic ring loop, tighten, sl st in 2nd ch. (8 sts)
Rnd 1: Ch 2, 2 hdc in next stitch, 2hdc each st around, sl st. (15 sts)
Rnd 2: Ch 2, 1 hdc, 2 hdc next st around, sl st. (22 sts) Finish off.
So I would do a provisional CO of 8, join them to work in the round, and KFB in each stitch around, then on the next round, (k1, kfb) around.
You can find instructions on how to do a flat circle from round blankets or shawls. Change colors when you want to. The only thing that might be a little complex would be where you see the blue crossing over the brown, which you can do by picking up a loop 2 or 3 rounds below. When you have the width you want, just start working even until you have the height you want, do an eyelet row, and BO. Then I'd duplicate stitch the veins on, and do an icord drawstring. Then go back to the beginning, remove the provisional CO and just run your yarn end around that circle of stitches a few times and pull it tight.