Sorry - I don't think making your own seat is a good idea. You could find out the hard way that it's not strong enough - severe injury/death in the event of an accident.

Manufacturers do a lot of testing before they sell their seats - the tests have been developed for good reason. For example, the seat and mountings will have to undergo a 25kN (2.5 tonnes) luggage retention sled test (I think it's done at something like 20mph) - that's a lot of kinetic energy the seat and mounts has to absorb/withstand.

You may think "so what - I'm not going to manufacture and sell the seats", but now imagine you've got a passenger behind you and you crash - if they're not belted up, and your seat cracks, the only thing stopping them is you and your seat belt - you'll not be a happy teddy sandwiched between the two. Alternatively, you bin the back seat and fit a harness. The harness fits through/over the seat. In a crash, your forward movement causes a downwards load on the seat from the harness - if the seat cracks, so will your back.

I'm not saying that seats can't be made from C/F, I'm saying that if you can't test your design, I don't think it's a good idea.