Guys, thanks a lot for all your valuable inputs. I decided to have a closer look at the stock fuel tank cap. I found the path of the fuel leak through the vent hole by observing traces of oil residue in the inner parts of the cap. It is going through the interface between the hard plastic fitting and the top inner surface of the cap, but definitely not through the one-way valve. I found a solution that cuts down the leakage quite significantly by inserting an o-ring between the plastic fitting and the top inner surface of the cap. I evaluated a few sizes of NBR o-rings (24-25 mm od and 1.5-2 mm thickness) and the most suitable one is the 24 x 2 mm. I have attached a few photos below and hopefully the installation procedure is self-explanatory. The recess in the cap made the o-ring installation very easy and straightforward! The cap has to be screwed to the tank snugly in order for the o-ring sealing to be effective, just the normal tightness would do.