My method works only when you're building, not when you have a finished plane, but it simplifies everything. I build in a vertical dowel at the recommended or intended cg, framed & glued to the main spar in a high or low wing, or into the belly for a mid-wing. Drill a pilot hole in the dowel for the appropriate size screw eye, and hang the plane a few inches off the floor from whatever you have overhead to tie into. Makes lateral and longitudinal balancing a snap for the first balancing act. If repairs or mods change your CG, or you just want to move it up or back at some future time, it's easy to readjust back to level, or let the nose or tail dip a few degrees, whatever you want to do.

For a plane already built, the suggestion I favor is to suspend your plane just off the floor with a rope/twine/strap/folded-rag harness running from fuselage just forward of the tail, through a ring (or could be a pulley) and back down to a convenient point at front of plane. Then hang a plumb bob from the same hanger point, long enough to hang just above the airframe. As you shift weight around, the plane will shift such that the changing CG will always be directly under where the plumb bob is pointing. Keep the weight/bulk of your harness light, since it is part of the hanging mass and gets included in the indicated cg.