Don't do the same as I did and assume the loss is due to syphoning when it may be a leak in the fuel system.
After getting a locking cap I still had overnight loss of fuel, coincidently I needed some work done under the hood too and took it to the garage, apparently when they started it up the fuel vapours from the leak trapped under the hood exploded, causing a full evacuation of the garage and
significant damage to my car. The garage was so pissed they refused to work on it and made me tow it elsewhere to get fixed. I ended up writing it off and scrapped it.
A
very costly error that could have been worse should someone have got hurt. I was told afterwards I'd been driving a ticking timebomb and was lucky to be alive.
The garage
still won't work on my cars, 8 years later