I can't supply the missing part, but can perhaps explain why it's missing:
1. You cannot get access to the flanges from underneath to spot-weld them together on a complete car. The scuttle and wings were joined as a sub-assembly before fitting to the car.
2. It's very difficult to include in a replica wing.
The missing part just fills in the gap. It has:
- a downward flange where it meets the scuttle, beside the screen.
- an external part which is visible from above/outside when the door is closed,
- vertical walls down the outside and rear edges
- a gutter from the base of the outside wall (but not the rear wall) which sits under the door and attaches to a horizontal panel underneath.
I would make the first 3 from one piece of 20g mild steel, then add the gutter separately, not least because this is how Rootes did it!
I recall that when I replaced my wings (in 1976!), I used glue and self tappers for the inner flange, with acces via temporary holes for a screwdriver in the outer vertical wall. You can MIG the rear edge, I think.
It should not be hard to make, but the forward edge will have to be joggled and later filled over where it meets the rest of the wing.
I could take a photo if you need it.