I solder casting balls on a few surfaces. One safe way - but time consuming - is to put solder on the surface first - then place the ball on top - and heat to solder again.
Try some paste solder. You’d still have to set up the piece with solder on top for gravity assist, but if you use range of solder types (ie. easy, medium, hard) and stagger your jobs, you should be able to accomplish your goal.