I hope this is in the right place here - not exactly a bag, but similar and for putting things in...
I've been wanting to make these for ages for my girls. They used to have hanging pockets on the back of the car seats, but then their legs grew and they needed more space. So I wanted to make something a little smaller, but somewhere for them to put all the bits and bobs you 'need' in the back of the car!!
It has one large space in the middle for sunnies, books, small toys and perhaps a small rubbish bag. Then two side pockets for a small drinks bottle and maybe a snack; then a pocket underneath for a foldable umbrella, and finally one in front for a small box of tissues!
My girls love them! and so do I! Nice to tick something off the to-do list too!
(I also posted a tutorial here http://jembellish.blogspot.com.au/2012/05/car-organiser-tutorial.html
if anyone is interested!)