For low pressure, or no pressure steam, you can butt the Tee section on, cut to fit the curve and pop rivet some little angles to hold it. Then a nice coating of silicone seal, maybe built up in 2-3 layered wipes. Welding is nice, but, out of reach of most of us.
Visit some boat building sites and forums and see what those guys use to steam canoe ribs. They've been doing that for a thousand years, too, about like bowyers. The difference is, you and i steam one bow and get real proud of it. A boatbuilder steams and bends in 50 ribs and then goes to lunch while the next fifty are steaming.
piper