Protecting PCBs from ESD charges outdoor
What are the disadvantages of this way? Are there any other ideas to
  protect the PCBs?


If the components really are ESD critical, Cardboard and most plastics are not ESD safe. Most materials are not ESD safe, because they generate static fields through triboelectric charging. That means your PCB moving around on the inside of the box could generate a field and still kill components.  However if you make the inside of the box you could make it ESD safe.

The way that this is overcome is to place ESD sensitive materials in a Faraday cage. If you have conductive material for a box, the electric fields will be zero on the inside of the box (assuming no internal charges exist, which shouldn't apply to PCB's). Anti static bags work in this manner by making a faraday cage with conductive coated plastic.

So you could use an anti static bag, or anything conductive, even aluminum foil would work, a metal box would work. 

The other problem is when you get the electronics out of the box, since then you would be introducing them to stray static fields. If you can't find a suitable ground (which would be unusual, because you must be powering these electronics somehow), you could use a metal table and ground a wrist strap to that, the most important thing is keeping all potentials zero.