Option before during or after :-https://www.fiatforum.com/ducato/357044-x250-cab-socket-upgrade.html

Self explanatory pictures.
heavy duty double insulated 2.5mm 2 core flex, 15 amp fuse and a few crimps

Thats how I did mine. Although I used an inline cable fuse, slightly quicker as you don't have to fix it to anything. And if you decide to fit the bigger battery it can't get knocked off when you are lifting the battery in and out.

That is the 110ah VARTA
The battery box is tapered so the fixed fuse holder is not an issue it also unclips quite easily. IMHO it is preferable to rattling around