Hello. I am new to this forum and the process of using forums in general. But that aside I am attempting to make a Helmet out of vacuum forming.

I have means to do the forming but I was wondering if there was any advice on making the mold for the helmet. I know past individuals have used replicas but as I am trying to save as much money as possible, I was hoping to make the mold myself.
What would be the process behind making a mold? Is is doable or should I just bite the bullet and buy a replica.
Thank You