Shouldn't need to be solid right? Seamless hydraulic tube is pretty nice stuff to start with and if you get lucky may only need to cut to length. If you can't find that locally, O1 or W1 drill rod and hollow it out. I'd be a little leery of aluminum. I would be afraid of it peening and end up loosing your tight fit. But, worst case you have to try again. Maybe start with 7075 T6, that is supposed to be similar to mild steel and is one of the stronger alloys. There may be another alloy that is tougher in compression though.
