You can get these already – looks more comfey than sleeping on the ground or in a car seat, but you likely need to be a midget or something to fit in there?
Inflatable Car Mattress Air Bed in Different Color with Air Pump, Repair Pad, Glue Kits
Amazon.com : Inflatable Car Mattress Air Bed Flocking + PVC in Different Color with Air Pump, Repair Pad, Glue Kits (DK-IB0FO-orange) : Sports & Outdoors
You can get these already – looks more comfey than sleeping on the ground or in a car seat, but you likely need to be a midget or something to fit in there?