Fuel filler cap leaking

[Ex-User (107751)]

ph2 172,
Petrol smell seems to be coming from the fuel filler cap when it's closed.
Also after overfilling the tank, I could see fuel dripping through the closed filler cap.
There is no rubber o-ring around the cap\filler hole , is it missing ?
What could be the problem , or what should I replace ?

 

[Ex-User (107751)]

Haven't found a solution yet.
Every time I park the car in the sun on a hot day with a full tank of fuel or even 2-3 gallons less, Fuel will start pouring out of the filler cap, and when I release the cap, a lot of fuel will pour out the filler hole.
So I avoid filling more than 2/3 at the moment, but still I can always smell gas from the cap.
I suspect it might be related to canceling the carbon canister and blocking the pipe from the fuel tank, as I don't seem to find information about any other vents for the fuel tank.
Also does anyone know what purpose does the box next to the big fuel filler hose serve, and if it could be related to my problem as it seems to be there for the case of an overflow?
The box is shown here: