Vikotnik, Holzverschalung wasn't it, it was a bit more complicated than that. Every profession has its own slang, I guess.
Hammershield, sheet styrene is out of the question, you'll never get this fine (barely visible, I know) woodgrain texture, so it will definitely be resin. Besides, I don't have the deep-drawing equipment to get decent results with styrene

I am not entirely sure what to do, though - just a few bunkers and tank traps, or a full-blown line of modular buildings? Don't know, really.
JollyBob, thanks!