I did have to modify my Fast helmet, followed a couple of YouTube videos fro reference, to get the mask inside it. Fairly simple process: remove screws from front of rails, cut plastic straps and carefully pull off front padded section, remove soft padding from polystyrene part and replace soft padding into helmet and reattach the rails. Effectively you remove the front padding to allow the mask space to sit inside the helmet, otherwise the helmet rests on the mask and there is this small oval gap offering no protection to the forehead. Think you need the FMA or Emerson type Fast helmet to mod it, not the cheaper china clone with solid polystyrene insert, although you could probably remove some of the insert to make space (depends on your craft skills).
I did actually buy a second Fast helmet, the type with the air holes in the top (maritime ?) as that may release more heat, however, it did not arrive before I wanted to go skirmishing so I just modified my original helmet. I can still use the dial at the rear to tighten it to fit.