I've been pepping for a little over a year now, made lots of Master Chief HD helmets, so I'm up to the challenge. I think pep is the best possible way to do this. Also, you don't need to make the backs of the body or the cog closed, you can just leave them open, or nearly open with just the edges pepped. It'll be much easier to assemble them and you can just pour some mud in, so no need for fiberglass at all. Once I pep the cog I'm going to mold it and make a solid onyx resin cast anyways (as long as that's ok with you, epilepticsquirl). So if anybody else doesn't want to pep one I can hook you up with a cast.
Again, if you're stuck on getting the file turned into a pep file, send dung0beetle (correct spelling this time) a PM.
That's an excellent model you made. I do think the bottom edges of the body should maybe be beveled though.