Evanix Rex P Fill Probe Leak

Greetings! I need to replace the o-ring (I found the specs already) on a Rex P that I got a couple of years ago. It developed a leak at the full probe, and I wound up storing it and never got around to fixing it.

Has anyone replaced the o-ring on one of these before? Would love to get some info before I proceed (I can definitely figure it out with trial and error, but if there's info out there somewhere that escapes me - I'd love to see it!)