I know this may not be exactly the info you were looking for, but this might be something worth considering.
First of all, DO NOT BUY THE COLD SEAT CAR SEAT COVER.
It is a great concept, but SO EXPENSIVE.
I looked at it, and was intrigued by the idea, but figured I could make one so easily, and that's just what I did. I searched on the internet for cold packs, just to see what sizes I could get since sotres don't have much of a selection, and I found these:
http://www.thermafreeze.com/store.html?page=shop.product_...
This is an awesome product! $9.99 for a 4 pack, which would do 2 carseats... read on... This product comes in thin, dry sheets. You run water over them to hydrate them, then freeze. And they can be re-frozen over and over and over. The sheets are 15"x10", which fit nicely into the carseat, one on the seat, and one on the seat back, to keep the carseat cool. And the sheets are only about 1/2 inch thick when hydrated/frozen, so they easily slide into my freezer on top of everything else I have packed in there!
I made a little pocket-pouch-carrier to put them in to keep condensation from getting on the carseat itself and to keep the cool contained a little, just a two-pocket fleece carrier, one pocket to hold the top sheet, and a pocket below it to hold the bottom sheet, with ribbons on top to tie it to the top of the carseat or headrest. Nothing complicated - I can sew a little, but don't use patterns, so I was able to make this just by winging it.
If you will be traveling for a while, I'd either take a cooler to keep the ice packs in to keep them nice and cool before you use them, or at least keep them in a shady area in the car, such as under a seat. When we get to our destination, I just slip the two ice sheets in the two pockets of the carrier, and set it in my son's carseat when we get out of the car. But even if you don't feel confident sewing a pouch to put the ice pack sheets in, you could simply place a towel on the carseat, lay the ice pack sheets on the towel to keep condensation off the seats, and wrap the towel over the top of the ice pack sheets, to hold in some of the cool. Even after a long day of the car sitting in the sun, the carseat remains cool, no more sweaty backs, and no more smoldering buckles. There's nothing worse than coming from a hot day at a park or something, and getting into a sweltering car.
I've even used the sheets on my own seat to keep it cool!
Hope this helps!