I researched this not to long ago to look into building my own, not that I necessarily need a Probox, I just like making things. I did however build my own version of the Profoto Hardbox.
From what I found the Probox has a 2 piece ground glass near the flash head, then a counter reflector with graduated reflective surfaces inside, and a plexiglass/acrylic front. All of which translates to near perfect even light output from corner to corner. I can think of other ways to get even light output across a 20 x 30 cm piece of acrylic, but it would be tricky to make something as small and convenient as this.