I googled this since I know what I would do, probably use epoxy on a personal bike, I shower in the stuff just love it. Apparently though you have some choices:

- Go to Chris King, he sells headset baseplates in a wide range of sizes! Nice headset, and you don't have to rebuild a fork, might be the one case where you save money by buying one of his sets.

- This is another one of my approaches, use a punch to dimple the seat so that around the dimples there are little rims that will recenter the part. Then use locktite or epoxy to fill the rest. I do this when making forks to center the tube. Unnecessary, but I like the idea of having the tube dead nuts with good braze clearance.

- Another option is a knurling tool, this raises enough metal that it may need recutting, so back to the machine shop.

- I also think that if the shimming isn't causing you problems that is a very acceptable route. As long as they don't shift out or something. Basic practice is you would single point something like this on a lathe, but once you get to very light passes that remove less than 1 thou, you are in reamer territory. I can't afford the reamer so I single point, however, in this situation, the reamer often doesn't do a better job than the singlepointing. Perhaps the reason is that best practice is to turn and then ream only the half a thou, but people are jamming these down and making essentially rough cuts. Anywho, when all else fails shim stock is the decent thing to do, so if it works be happy. That is what a machinist would do.