Referring back to my question here: Can you get nuts that attach to regular screws?
You can see that I have a metal pole with holes either side designed to be wide enough for a regular wood screw. It was recommended that I drill the holes wider to fit a bolt through, but can you get a thinnish bolt that'll fit through that hole already?
I drilled the hole out so that it's 5mm wide, and the bolt would need to be about 70mm long I reckon to include enough excess to have a couple of nuts attached. I also wonder whether I should use washers on either end to spread the pressure across a wider area of the plastic sleeve.