Are you gonna use it as a coupler or something else.
Maybe get two pieces and join them with a loc stiffener inside or a piece of bodytube outside?
Balsa Machining has 3" coupler listed, they might have something bigger.
I have been thinking about this too. I suspect that I'll need to cut two couplers. Otherwise the saw kerf will prevent the coupler from being round. But I hadn't really thought about the structural issues that this could cause. If I'm going to do this, I definitely need to design something to compensate for the lost strength.