The original shapes were foam. Then I coated them in primer but squadron putty would work for that too.
I then made a mold off of those shaped in plaster. I waxed up the plaster so that nothing would stick to it.
Then I mixed up Magic Sculpt epoxie putty and rolled it out like a pie dough. The real trick here is to roll on parchment paper and use talcum powder like you would flower. I rolled with a spray can of primer. The lip on the edge of the can made the whole sheet uniform.
I then pressed the sheet of epoxie into the plaster mold. After the epoxie is set you have to trim it out carefully. I put the rough trimmed parts back into the plaster form and sanded the seams flat using the mold top as the reference. You can really finnesse the epoxy shell with sandpaper.
The scribing was done with a broken sewing needle put into an X-Acto handle. I used the Hasegawa scribing templates.
You have to be carefull with the epoxie shell though. They are very brittle.