Just got off the phone with Performance Cat. Their wings for the nacra 450 would fit on top of the beam, not inside. They bolt attaching brackets to the beam, which attach to the tubing. Tubing is double sleeved. Price around $1000.
All this concern about bending or welding round aluminum tubing makes me wonder if using straight pieces with bolts would be simpler. Could even use angle or channel material, maybe even stainless instead of aluminum.
So I've attached a drawing of what this might look like.
