Hey B Merrit, (first name would be good)
For a one off rod trench in a finished neck I just use a couple of strips of ply and a template collar on a lam trimmer. Attached are some images of one I have done just recently. You will notice that this neck is heelless which does make life a little easier but the same technique can be used for a heeled neck you just need to clamp it so it will sit still.
As you will see the truss rod in this neck had been installed off centre and the rod was just a piece of flat bar laid on side anyhow. I remove the FB and this is what I found.
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I removed the old rod and cleaned up the FB surface with a sanding board.
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I set up the strips of scrap ply so I could rout out the original truss rod trench and reposition it in the centre of neck shaft only the slot would be wider and much deeper than the original. The scrap ply is carefully positioned and held in place with double sided tape and a few panel pins punched below the surface of the ply.
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The new wider deeper trench was then refilled by gluing in a fillet of hardwood (local jarrah) and the neck was set up with the ply once again only this time stops were added at each end and the gap was narrowed so the new trench would be a snug fit for a two way adjustable rod.
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With a bit of clean up the new truss rod was installed.
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And then a fillet was glued in place and leveled and a centre line marked ready for a new FB to be glued in place. The old FB can be seen to the far left in the next image, it was bent like a banana.
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All done

Whoops, just realised that the last image is actually of the glued in fillet before the truss rod trench was routed. Can't find the image I want to post right now so that one will do

I'm sure you get the idea
Have fun.
Kim