I realize this is an old question, but here's an alternate solution:
You have a 5" wide piece of plywood ripped at a 45 down the middle. You want to have the nicer side facing away from the wall since any cleat not in use will be visible. Call this the "bottom" half of the rip.
Instead of flipping the "top" half over so its bevel is ...
A few search's led me the term "plastic stacking corners" OR "box stacking corners".
Here is one example.
There are several kinds of those but i was not able to find metal ones.
I did find a metal corner that is not what you have referenced but could work.
It seems you could also use a piece of angle aluminum fastened around the the ...