It's just White PVC I think! The box is made from wood so he glued a load of that stuff inside...
You only get the reflectiveness when shooting at a small angle, when shooting from above you don't get it. That's one other tip too, get your camera on full zoom and reposition it to fill the frame. Full zoom will reduce and distortion, well with mine it does as it goes all the way down to 15mm.
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