First, get the top of the forearm true. This takes a long-sole (10"+) plane set for the smallest cut possible - about .001" - operated by someone who knows how to sharpen, adjust, and use it. Lacking that, use a long hardwood sanding block with finest-grit garnet paper. Once done, rub soft artist-grade white chalk on the true surface and leave the handguard bare. Press and rub together in position, then use a very sharp cabinet scraper to work down the high spots where chalk has transferred. Repeat until chalk indicates full contact. (Same process as fitting the bass bar on a violin. See
Derek Roberts Violins - Fitting the Bass Bar for photo tutorial.)