You can get these connectors online from China. I've seen it a few times but not sure if it was on AliExpress. I guess so.I recently bought an Audiophile Ninja cable for my D8KP and it seems there's 3.5mm plugs with the locking mechanism already, I'm not sure if they create'em themselves but the locking thing is made of plastic and you can tell it is not an HQ 3D printed version but it works
I'll look around and see if I can find'em, China probably makes these already
Other solution is buying a stock cable from the lower tier sonorus headphones and rip the connectors from there. I've seen a few on the 2nd hand market and they were like 20/30€. Not that they are easy to find, but some pop up from time to time.
Meze 3,5 connectors from their cheaper cables also work with Final models.