make sure that your helmet is reinforce if not it will crush it...i would be careful with some of the brushable rubbers they tend to bond to your model and you'll have to destory your model to get it out
so make sure to paint your helmet glossly and get some of the Ease Release™ Release Agents from smooth-on and spray your helmet before moulding and also spray the inside once you get done and start casting helmets up if not it might bond to the mould itself.....i had it done for me, i tend to try everything out to see what works for me