I think another point I hadn't included was the flexibility I had with prototyping different boxes. The hardware was changing all the time, and being able to just 3d print another revision was very convenient.
If I did something like that with styrofoam, it probably would have gotten pretty expensive. The precision you get from printing was also a plus, as there needed to be slots for cameras, antennas, and circuit board mounting holes.
You can get a 4' x 8' sheet of 1" EPS insulation from Home Depot for under $20 (thicker available if you need it, costs more) along with compatible construction adhesives.
You do have to be careful with glue selection because a lot of solvents will dissolve it, but there's plenty that will work.
What's expensive? You can buy a medium sized cooler for $5 at Walmart. That's a fair bit of foam.