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You just put any of these pieces on the top of the chamber. My suggestion is to use a broomstick for this. You drill holes where the bolts go. You can take a marker and paint on top of the bolts and then lay the broomstick on them = you will know where to drill.
Sand the broomstick flat on the bottom against the chamber, put rubber gaskets around the holes, make room for the rivet as well on the underside. => This is to reduce the pressure on the rivit in the middle of the chamber.
Now you have two handlebars and you can stretch harder and bundle more.
I like to see this action. Have you tried this yourself?