Hollow inside with space for electronics. I printed in clear transparent plastic, masked off the parts that light up, spray painted the rest black. Installed LEDs, a switch, and 2AA batteries.
Note, this is the chest box, only. The chest armor it is mounted to in the photo above was made from a 20-liter garbage pail.
Feel free to tinker with the model:
Top part requires a brim and supports to print properly.
The partly-visible helmet is here: https://www.thingiverse.com/thing:3124596