You could get a decent half-round file as Chrome Molly suggested, and you won't need any shims because you'll be able to get a very precise fit quickly.
This is aluminum, so a coarse file might be best.

Alternately, a piece of coarse sandpaper wrapped half-way around a standard seat tube should produce a proper radius by sliding your derailer up/down along the sandpaper covering the tube. Should cut very quickly depending on grit selected.

Perhaps use tape first to adjust diameter and protect paint as needed while sanding.
And I hope it goes without saying that the grit faces away from the tubing!