I went to the Christmas Tree Shops and spent $1.00 on a small foldable, fabric storage box that had a handle on one side.  I have an SUV so there is not a ton of space in the backseat and the box is big enough to hold all the dolls, books, mini toys and blankets that my kids like to have.  My youngest has a Britax carseat and on both sides it has bars that go around the seat (if you have one then you know what I mean). To secure the box so it won't slide around, I took some velcro strips ($1.00 for a pack) and weaved the strips around the handle and the bar so it was attached to each other.   The only downfall is that you don't have a place to hold cups but my older daughter already has built in cup holders in her booster seat so she doesn't have that problem.  But for a total of $2.00, you have a cheap way to organize toys in your car that won't slip and slide everywhere.