Several options. Epoxy is one, Ptex candle or melted plastic filler is another. Also you can go to a metal insert and use a machine screw. If you just pulled the plastic insert out, first try putting it back with plastic rated epoxy. I had a metal insert spin in my Ducati's airbox and this fixed it up perfect.
FWIW, beefing up the shroud attachment with an upper mount to the rad guard/brace goes a long way in preventing this from happening.