I absolutely love it when people make functioning 'things' out of costume and cosplay props, so this is fantastic.
From first-hand experience, Apoxie Sculpt is magical. You're gonna have a good time with that stuff. You'll end up mixing a lot more than you need the first few times, but you'll eventually be able to judge how much a particular detail or shape is going to need.
Another method you might want to try is just using styrene sheet for flat details and sharp angles. It's just a lot easier and will end up being cheaper than using a ton of your Apoxie Sculpt.