This is the 5x3x2.5 foot box Spiers mailed himself in:
The journey took over two days. Spiers stuffed himself in the box with some "tinned food, a torch, a blanket and a pillow, plus two plastic bottles - one for water, one for urine." You can read more about the whole ordeal here.
That is because the eyewitnesses were told that it was impossible, and that they must have been mistaken.
That film was crap in every aspect anyway. Saw it in the 90s and even then it was cringe.
These might not be life jackets at all, but more like what we do with seat belts and airbags now.