The solution to all of these problems: make a mold and cast a copy in resin.
A while back I started making the mold:
As usual for me, the jacket mold was silicone rubber from aeromarineproducts.com. The plurple stuff is the pourable version while the pink stuff is the brushable version:
If you noticed the ice cube tray full of silicone in the above picture and wondered "WTF?" good for you. I was forming blocks of silicone to use as registration keys. These were embedded in the second-to-last layer of silicone so that the rubber jacket would have nubs sticking out to keep it properly aligned within the mothermold.
Once I had enough layers of rubber to make it strong enough for the strain of casting, it was time to lay up the fiberglass mothermold. Here you can make out the outline of the rubber blocks from the ice cube tray:
Once the fiberglass had set, it looked about like so:
The mold on the right was for my Warhammer 40k Chaplain helmet.
Before pulling the two halves of the mothermold apart, I drilled holes around the flange that sticks out around the helmet so I could bolt the two halves together. Then I unbolted them, separated the halves of the mothermold, and removed the prototype.
Casting copies of something hollow (like most helmets are) involves a process called rotocasting or slushcasting. Basically you mix a small batch of resin and pour it into the mold. Then you rotate the mold so that the resin flows over the entire surface and leaves a layer of material throughout. Once the resin cures, you repeat the process a few times until you have built up enough thickness for the part to be strong enough to use. Then you separate the halves of the mothermold, peel off the rubber jacket mold, and marvel at the shiny new helmet you've made.
Repeat as necessary until you have enough helmets for your whole army.
Here I am pulling the first casting out of the jacket mold:
Here's the first casting next to one of my MkVI Master Chief helmets:
Once I had that cast cleaned up, I gave it a quick coat of blue paint and it promptly fell off my list of things to do. To keep it out of the way, I hung it up in the rafters with all of the other unfinished helmets.
Then, months later, I got an email from Adam Grumbo, founder of the 405th Infantry Division.* He was working on a rushed hush-hush project and in dire need of a MkV helmet such as Caboose wears in seasons 6-10 of Red vs Blue.
I pulled the blue helmet down from the rafters, knocked the dust off of it, and mailed it to him. Some time later, he sent me a snapshot of it all painted up and it looked good.
Then I forgot all about it.
Fast forward a few months, I'd bought the DVD of Halo 4: Forward Unto Dawn. Mixed in with the special features was a live action Red vs. Blue skit by Rooster Teeth. At some point in the skit, in walks none other than Private Michael J. Caboose wearing a very familiar helmet:
So now one of my helmets is famous. Cool.
It was enough to convince me to grab the mold, pull another casting out of it, prep it, spray on a coat of blue paint, and hang the unfinished helmet up in the rafters again.
I'm sure I'll finish one of these sooner or later.
*The 405th is the central costuming forum for all things Halo-related. Check it out at 405th.com.