The way we have been doing large pieces of geometry is foam sculpting. We use the white insulation foam from Home Depot/Lowe's (the pink / blue stuff is good too, we just don't have access to it) layer it up to the appropriate thickness and height then carve at it with a knife.
In order to harden it you can use a variety of methods. The most durable is sealing foam with masking tape, covering in fiberglass matte, then filling in with bondo. It's time intensive and some people are intimidated by fibgerglass.
You can always use plastidip, paper mache, or just straight bondo but so far I have found that none of those are nearly as strong as doing fiberglass.
But case in point the girl in our Monster Hunter gorup on the left with her giant fist hammer is a foam sculpt.
Edit: The hammer is foam sculpt, the girl is actually a girl haha
This one is just foam coated in plastidip. Not very strong but it covers the foam texture quite nicely.