Joy,
You might be envisioning an extruded-type aluminum angle iron
with the standard 1/8 inch thickness. That's not what I'm after.
What I'm thinking of using is flimsy aluminum flashing no thicker
than a few sheets of paper (haven't measured actual thickness
yet) that I would crease and fold over into the "L" shape.
I see your point tho, and what I'm considering might still be a
bad idea. There's a whole lot of stuff happening back there at
that junction, as your drawing shows. I'm still investigating
the situation and have yet to settle on what I'm going to try.
Jimbo / '76 Sovereign