Try this link -
http://www.marinepartdepot.com/newststdehi.html You'll have to check diameter to be sure their stuff fits your rail. I'm planning to modify my bow rail to open a gap in the front and will be looking into it at some point. I got a couple of turn latches for my back hatches from these guys - their prices were good and the latches look fine. I like that they"re stainless and not pot metal that salt water eats alive.
Don't be fooled by their "free shipping expires tomorrow" nonsense - it starts right back up.
I don't think any of the bow rails are aluminum, but I'm no expert.
Good luck with the project & please let me know what you find that fits.
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Scott, Portland, OR '85 V-20 I/O, Merc 170