So I want to make a custom fiberglass or preferably carbon fiber rear diffuser for my car. Since no one makes them I guess I'll have to do it myself. I have only a vague idea how to do it though since I've never worked with either before. I am good with my hands and I'm a fast learner though. So, how the heck would I go about doing this?! Any ideas or websites where I can get some info?

CF is difficult to work with and quite expensive... you could always look for a blank sheet of CF and cut that to size. I would ask someone to see if u could fit the APR splitter in the rear. That, or talk to doc Isotope about making an abs one...

Ya you won't be able to CF. If you got enough money I bet Kaminari or maybe APR could whip something up for you.

You can CF its just a PITA. Abs or FG are your best bets for making something functional at a reasonable cost.

Last I knew CF required special equipment...

you can overlay CF and then put resin on top... it just would be a PITA... but to lay it correctly you need to use a vacuum with a fiberglass mold.

My buddies and I overlayed a few skateboard decks in CF when we were bored at the shop... weave wasnt that tight... but sure as hell looked cool

It's not worth it, if you never work with CF, you need lots of experiences for large project like that, you'll spend alot of time making it and waste lots of money on materials and equipments.

Do it in fiberglass first because CF is to expensive to test around with. Start off with a trip to autozone. Fiberglass sheets and regular resign.