REALLY BIG HELMET!!! 
 
 
I added a length of half round plastic strip around the opening and sprayed on primer/filler. 
 
I did some test fitting and decided that the Helmet & Fink Head were too large. 
 
 
 
I laid 2 strips of 1/4" masking tape on the helmet, one from front to back, the other from side to side. I traced the 2 lines, removed the tape and cut the 2 pieces out of the helmet. 
  
Then I glued the 4 sections back together. 
 
 
 
This should be the perfect size...