The best way to create a form of any kind is to cut 4" to 6" thick pieces of foam to the basic shape and then laminate them together with foam to foam glue or contact adhesive. You can also start out with a thicker block of foam, cut thinner pieces of the same or firmer density and glue them to the block. This way if you want a bolster area that sticks out further than the middle area, it is much easier to do than to try and cut the shape out of a large block of foam. While using an electric knife would work, there are better tools for the job. These Bosch foam cutters have a lot finer teeth that cut with a lot more precision. The problem is that these cutters are very expensive. You could also use an electric fish filleting knife that you can buy at Wal-Mart for about $20 to $35. Whatever you cut with needs to obviously be sharp or the shapes will not be consistent.
You can buy foam in any quality and any density you want, but the firmer the foam the easier it will be for you to cut and the better it will hold up to use. You will need at least 80 density foam with 90 or 100 density options to consider. I have access to up to 150 compression foam through one of my suppliers, but that's way too hard. Foam is sold by a four number system, the first two numbers are the quality of the foam and the second two numbers refer to the density. 3580 foam is better quality and twice as hard as 2240. You should also use FR foam (fire retardant) for a car seat.
No one lives forever, the trick is creating something that will.