you will want to resin and fiber-glass that if you want it to last longer then several uses and wearings. Cardboard WILL NOT last long by itself for a costume.
Bondo body filler is what you use to add in details, put some on, sculpt it a bit, wait for it to harden, then sand it out into the detail you need.
For now, what you have is, while very good, is not by itself all it can be. Trust me, if you go the full approach, it may take a lot more time and effort, but in the end you will be stunned by the awsome costume you will have.
PS: two layers of fiber-glass=protection from baseball bats