As you said run perforated angle iron from the center stile to the next stile over.
Mount garage door opener off center.
Use one piece as near to the top as you can which would be just below the strut.
The rail ended up being about a foot and a half off center.
My garage door opener came with the following instructions.
I mounted one off center for a door on my own garage that was 16 wide and it worked fine.
To offset mount the opener arm to the door section.
Make sure there is a strut on the top section it s required when using an opener run a second piece 6 to 12 below it.
The opener can be installed within 4 feet to the left or right of the door center if there is a torsion spring or center bearing plate in the way of the header bracket or door bracket area.