Getting live wire through into engine bay? (split charging)

Hello guys and gals!

I'm wanting to fit a leisure battery into the boot of my mk3 superb SE-L.   I've already got the battery and the split charger, but I need to get a wire through into the engine bay to connect to the battery.   I've been upside down in the footwell trying to find a grommet or something, but does anyone have any suggestions? 

I saw some lovely fat beefy wiring coming through for the media system behind the glovebox, but I'm thinking it's best not to play with that hehe.

Fairly okay with electronics, so just need a hole.   Many thanks guys!