There is no particular requirement for a purpose-built mixing elbow. On my Cal
36 there is no room for an elbow so I have metal exhaust
tube for about 18 inches from the engine to the water
injection point ... it loops around over the engine and to one side. After that point it's rubber exhaust hose.
The problem is the heat from the exhaust tube. It's wrapped with motorcycle exhaust wrap, but is still very hot.
An advantage is steam from the exhaust, after shutdown, probably will not reach the valves and cylinders as it tends to with engine mounted elbows - causing rust.