To take it to a machinist and have him set up a mill or a flycutter operation to hog the material flat - is also going to require that the back of the radio will have to removed along with all of the electronics from it as well. Then spot, drill, and tap holes for the external heat sink which - by the way - is not a real improvement over the original part. The machinist doesn't work for free and will charge for time. Net benefit? A much lighter pocketbook with little to no real thermal change.
Better to just install a fan in terms of time/cost/efficacy benefits - IMO . . .