You may want to consider fitting your visor to your helmet during your bondo stage. Get the helmet pretty well smoothed out, then figure out and cut your visor, then cover the visor with masking tape, insert it and temporarily secure it, then use bondo or putty to fill any gaps. Once the bondo or putty hardens, remove the visor and smooth the remainder of the helmet, and paint. This way, when you've gone through all the trouble of making the helmet look great with paint, you won't have ANY gaps when you install the visor.