thanks for your question. For me, the easiest method for making the mold is using an old helmet (skate, etc). It's got only a few holes and after removing a lining, the volume is slightly bigger.
Seal the holes, put some plastic bag or a kitchen wrap in it and pour the plaster. After cured this is your mold. Some sanding AND more volume is needed.
I think it might be possible to use a spray foam (or 2 part PU foam), in a similar way instead of plaster.
The plaster mold is heavy tho, very good for laying up. Light mold would bounce all around.
I am using a silicone swimming hat to seal the plaster before the layup (vinyl wasn't working).
The swimming hat's might also be used to accommodate a volume or size. Wrap a mold in 10 rubber hat's and hence the bigger size
How bigger one needs depends on a foam used for protection inside a new helmet. I'm using 5mm foam. My skate helmet was too big for me, but this was perfect in the end because it can accommodate this 5mm bigger volume.
I did look at commercial molds but couldn't find any. There are some modelling styrofoam molds on e b a y but can't find the right sizes. Good tip thanks.
Even if a skin helmet is slightly bigger (I am 59-63cm helmet size), one can always use a ticker foam for a smaller head, resulting in a better protection as well.