It seems that the more I make, the less I know.
Things that seem to be quite straight forward turn out to be not so easy in real life. Live and learn.
So, I made the body of the Trebuchet and it looks a bit messy. To clean up the look, I think I want to put a chamfer on the edge of the hole. I would like it to be an even 2mm all around.
Looking at it, I think I have concluded there is no way I can do this with clever tooling or such. The only way I think I can do it now is with the little deburring tool like this one:

Or use a Dremel but then getting it an even 2 mm all around is going to be tricky.
Any better ideas?
This is a picture of the part as it is now. I know the surface is not what it needs to be yet, that will come.