Didn't notice the pics at the bottom of your first post until now. Looking at the design I think your best option would be to use some PVC piping as used by the guys that dig up our streets (someone mentioned it earlier). It comes in various sizes and its the kind of thing you can find just lying around the streets *thinks back to a uni project requiring some*

It would fit nicely into the design and would happily take the dents. It won't be nice and shiny but then again you can always throw your original piece of ali over the top.