Re:
I made a seat-post one for a mate.
Just cut the can to rough size; scissors work well. Then bend as many as fits best around the seat-post to fit in-between it and the seat-tube.
I then bonded (metal-two-part adhesive type stuff) the two 'shim' pieces together and left to set wrapped around the seat-post, wrapped tightly with rubber bands!
You can then anneal the top and 'bend' it over a bit to give a lip so it doesn't drop down.
It works well and still going strong on his bike
Not sure of the can thickness, but I could measure for you if required?
Cheers
Pics!...