A lot of cue makers are using a CF kind lately. I don't know who makes them, but it's basically a 3d printed adapter with a nut/metal threaded insert in the middle. There is also a bumper on them and most makers 3D print their logo into the adapter part. This screws into the end of the butt, which is threaded. If the customer doesn't want an extension they just put a regular rubber bumper in, if they want one they screw the adapter in and it's there full time. I've seen them in 6" and 8" but again, not sure if someone is making them for cue makers or they are making their own.
The CF is light (obviously) so it doesn't really mess with the balance point too much.