I've been looking at car seats for Daisy and Beau with the following considerations:
1. Big enough for them to comfortably lay down and sleep
2. High enough sides that they wouldn't "slip out" in a sudden stop
3. Secure seat belt system
4. High enough for them to see out
5. Wouldn't "break the bank"
6. Two seats need to fit in our Prius and also in the bucket seats in the back of our Yukon
Most of the ones I liked were made of foam, but the inner area wasn't all that large because of the thickness of the foam walls. I need TWO - so two of the largest wouldn't fit in my Prius. Many were very expensive....cheaper ones didn't look as secure.
While searching the internet, I found a new car-seat that became available in late April: http://www.solvitproducts.com/on_seat_booster_main.htm
It has a heavy duty, rigid frame that snaps together and a cover that zips and velcros on plus a pillow cushion. There is a "D" ring fastened to the frame and has a safety leash that slips through the cover that you can attach to a harness. The car's seat belt goes through the frame to fasten the seat to the car...kind of like a baby's car seat. I'll try to attach some pics (if I have them re-sized correctly)!
Can of Campbell's soup is to provide size scale. The bottom pic shows the opening to place the seat belt thru...
$64.99 each on Amazon (free freight.....)