there're a lot of factors you need to consider using carbon fiber especially making a custom case. The thickness would be the least of my concern when it comes to carbon fiber, it's strong enough for anything as long as you lay it out properly.
1. There are different weaves and they have different properties, such as unidirectional carbon fiber which is rigid in one direction and flexible in another.
2. pre preg or resin infusion.
3. carbon is conductive, it's going to short components if bare pcb is touch against it.
That's all I can think of right of my head. the conclusion is it's not the size matters, it's the way you use it. If you're just seeking for the looks, I recommend just going with a wrap just because it make life so much easier.