I just get 4 pieces of styrofoam and cut them to side and slide them in the box. Basically a box within a box. Easier than finding pre-made styrofoam pieces that look like that.
Check out Home depot or a home improvement store of the like. They sell packs of styrofoam that I use to do this same exact thing. Over near the inslation things. I think there are like 4 or 5 sheets in a pack.... 4 feet by 15 inches or so... just cut to size. Works perfect