I'm not sure if there is any smartass white on white text in this question but I'll take it on face value and give the best answer I can.

A simple solution would be to add a sleeve to the tack end of your existing boom to get the extra length.

If you want to go to a new boom I guess it depends what type of tube you have available in ali or carbon. The filament wound tubes are strong and what we use in A's. 38mm OD for end boom sheeting and 43mm OD for centre sheeting. That is with a 1.8m foot. For a longer foot you'd need to go stronger.

For internal sheeting you probably need bigger ID than those to fit the blocks comfortably. I used internal cascades on my A's in the 80's (we called it an archemedes system) using a 50mm OD 1.2mm wall ali tube then rolled flat on either side to give it about 55mm x 45mm OD. The beauty of this meant the blocks all ran in one plane because they couldn't fall sideways and the boom was stiff in the vertical plane where needed. I had a local supplier who did this section quite cheaply. The advantage of carbon is you can downsize it over ali and get the same strength. In this situation you need the size to fit the blocks so no real advantage in going carbon. There's probably no weight saving and it will almost certainly be more expensive.

Cheers
AUS9