Here's the almost-finished library bag/car organizer based on this tutorial from Katie at Punk Projects. Thanks, Katie! Gotta find that button foot so I can finish it up - buttons go where you see pins here.
This view shows you a little more how the strap goes around the passenger's seat. Depending on your tolerance level for floppiness, you might be able to skip this strap.
Here's what I was trying to explain about making an outside-pocket bag out of a single strip of fabric. I'm not good with adding text, but maybe this will help.
E-mail me if this isn't clear. Honestly, this is one of the fastest bag-making methods ever.
Here are the produce bags we use. The check-out folks at our local Wegmans have never had a problem with them, and if they get sticky, they wash like a dream. (the bags, I mean. I assume the same's true for the Wegmans staff.)
ETA on 7/27: Thanks to listener Elisa, we now have the link to this bag tutorial. Here 'tis. Thank you so much, Elise!
Finally, here's the link to Martha's Beach Towel Tote. I think the basic idea is really good, but I'd definitely stitch sides into it to make it more reliable as an actual bag. Let me know if you make one - I'd love to feature your photos and thoughts.