I got a simple carving set, I bevel the inside edge smaller than the outside ever so slightly then I trace the outline of the lockplate with a marking knife. then I cut it out and carve it down. then I scrape the interior surfaces flat then I push the lock in and use a dowel on the body of the lock plate tapping it deeper then removing the lock and scraping the contact away.
If it is a flat plate I will snug it up to the barrel then carve away the standing wood to a point then sand the metal with the wood, the result is a wonderful fit.
My method more than likely is not the best, quickest, or safest way to do it, but that's what I do.