Truss Rod replacement help

[Useless amateur]

Ok I have removed a truss rod and will buy a new one to replace it, does 580mm sound right? I haven’t searched yet but that appears to be the existing one. 
What I want to know is IF I can’t cut a fillet and replace the removed timber from the mech is there another material I can use? I might be able to cut the fillet after routing the hole to smarten it up but my hands shake a lot so it would be easier if I could use something simpler. Any help appreciated. 
the photo is as it is now pre routerered to tidy it up

Jon