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I had my dad turn several of these out over the years, and they work well.
He has made them to the specific handlebar that I was trying to extend, so each one was made to be tapped in with a snug fit.
They were also not made to extend more than two inches.
That way the grip portion still covers a section of the original handlebar.
The main objective was to extend the bar just enough to be able to install a brake lever on the grip section of a contoured handlebar.
The hollow was to allow the bar to breath and also not show if a grip with a breathing hole was installed.
I've always thought they could be easily made and marketed, but I'm just not that guy.
I will say, that I have been using them for years, and have never had one fail or pull out un intentionally.