With our heat wave, I decided the little woman needed a car seat cooler. I found a picture of exactly what I wanted, but she didn't provide a tutorial (she's now selling them herself, so I don't blame her!). I did study her pictures and come up with a design that would work for our car seat. It's basically a strip of fabric with pockets that each hold an ice pack. I freeze the ice packs over night and bring the whole thing with us on our day out and about. Whenever we park the car, we put the ice pack in the seat to keep it cold while the rest of the car cooks in the sun. When we return, the ice pack comes out, and she gets in a cold seat. SHE DOES NOT SIT IN THE SEAT WITH THIS UNDER HER. It's either the ice pack or her, never both : ) Just wanted to put that disclaimer out there.
It's a few layers of fabric sewn together with bias tape around the edges and pockets on one side.