Curious as to why it has to be carbon or aluminum?
In any case, special stoker stems tend to be longer than normal stems, and there are even adjustable stoker stems, such as this aluminum stem from Zoom that can extend from 155mm to 200mm:
https://www.universalcycles.com/shop...2&gad_source=1
Or this one from ControlTech that can extend from 190mm to 230mm:
https://www.amazon.com/Control-CONTR.../dp/B0CJY7KHYG