This bag was made based off of a pattern from Shavasana Yoga Mat Bag. I liked her pattern, but wanted something more see through. I changed the body of the bag as follows:
--This now starts after you have finished the base (I did the base with hdc)--
Ch 3, *sk 1 hdc, dc ch 1 in the nxt.* repeat from * around, join in 2nd ch of ch 3.
::Depending on how many stitches are in your origional base circle you might have to fudge it a little bit here. For instance in mine, I had to skip more than one stitch to reach the 2nd ch.::
Ch 3, dc ch 1 in the very first possible chain space. Contine dc, ch 1 in all the chain spaces around. You will be working in the empty space, not actually in a stitch. When you get to the chain, don't [dc, ch1] into the last space, but skip it and sl st into the 2nd ch st of first ch 3.
:: This helped me from the bag increasing in size or shaping itself weirdly, but you can alter the joining as you see fit. ::
Continue in this way until the bag fits just short of your mat once it's rolled up. I kept my mat next to me and fitted the project over it every few rows. Actually pick up the mat once it's inside the project, that way you can see how much it stretches.