Watch fans, Omega tool?

I’ve a Omega seamaster auto chrono. Since a service 2 years ago it’s run consistently fast by 20 secs per day pre the service it was v accurate. I’ve sent it back 3 times, they keep it for 3 months and still fast. It only runs fast on the wrist, it’s accurate if i don’t wear it!

So, I’m keen to have a go at regulating it myself. Is this daft? There are some good guides which suggest it’s not too hard. But I need the tool to unscrew the case, and struggling to find one. Anyone know of a good place to look?

Thanks