That follows along the lines that I would normally use for making multiple objects, except that I usually make a GRP female mould. I've been this route many times, with plaster or GRP, and both are fine. If the shape is more complex, or you plan making many copies, GRP is better for the mould as it is more hard wearing and will last longer. The mould for a helmet should be about 1/4 inch thick to maintain strength and shape. I've also used the bike valve method before.
You can mould quite complex shapes by making the mould in more than one part, each section bolting together for laying up the mould, then taking apart to make releasing the shape easier.
For the helmet I just made, I wasn't planning on more than one, so I dispensed with the mould stage, and went straight for laying up the GRP on the helmet and finishing the outside. This method is quicker and cheaper for one off's.
Has anyone else made themselves any accessories because originals were hard to find for example, or for weight like the helmet?
Steve