I redesigned it several times in my head - how I would attach the zipper, how to keep it stiff, etc. Then one day I was out at a quilt shop picking up a new ruler and I spotted 3 yard zipper tape - perfect for the main zipper on this case! And at the dollar store I found craft board (someone had suggest the corrugated plastic for sale signs, but I couldn't find them. Craft board (foam core) seemed perfect instead. So now I had the supplies, it was time to get started.
It took me about 3 days, and 4 broken needles, but this is what I came up with. It carries my mat and 2 long rulers on the outside. And has a pocket inside for every ruler I own. I think the hardest part was figuring out how to sew all the pockets together in stacks (from the top one down!). There are a couple of things I would do different next time (and will change when I write the pattern for this). But it came out just like I was hoping and is perfect for carting around all my rulers. The 2-way zipper goes all the way around 3 sides. Each pocket has a snap closure so rulers don't slide around in the bag when it is lying down for transport. And the handles wrap all the way around the bottom of the bag for extra strength and security.
It is however quite heavy (although some friends commented that, given what it is and how much is in it, it's much lighter than their options). After taking it to one sew day, I quickly determined that shorter retreats need a lighter option, to carry just a selection of rulers required for that single day's activities.