I was thinking the same thing when I read that. Back when I had my shop lathe I would have just turned one with a threaded end and installed a nutsert in the top of the door and screwed it in with a drop of Loctite.
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Ralph they ran me out of Louisiana too. I am in Florida now. Mine being gone doesnt really seem to be an issue. The door stays shut fine and there is a button strap to hold it open while at sea. If it is a chore to repair I will probably let sleeping dogs lie.
So I looked a little closer at the shower door today...there's two screws up top which make it look like you may be able to pull the top rail off the frame and get to the bottom side of that post and repair it or install a new one....
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The aluminum would probably be too thin to hold more than one or two threads. You would need a nutsert it looks like a rivet with internal threads in it for just this kind of situation.
I had my shower door apart. The panel was only held in with a vinyl spline but it did break when I took it out. I replaced it with clear lean.
Walt Hoover