Don't even consider carbon, it is just to brittle and will snap with our 90 odd kilos landing on it. Plus changing from rectangle to round is a lot more hardware than what you first think ( gooseneck for one )

Do what I did and just rivet two small SS rig adjusters to the end of the boom and mount the out haul pulleys through that. The Ali boom itself is long enough to cope with the sail it is just getting enough length to mount the out haul pulley ( I think mine only needed extending by about 30mm ).

In my copious spare Ali tube I have a 50mm x 1.6mm length just suitable for a boom, lets talk when next at Datchet