Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:4240677.
Earpad adapter ring for Sennheisers HD5X9 series.
We also know that they're compatible with the HD380 Pro, thanks to Tygerr for testing them.
Two files are included, a replica of the ring that is used with the stock pads, and another one that is 5mm tall, which will fit replacement pads like Brainwavz a little bit better.
Pictured above is a spare pair of Brainwavz earpads that I had previously cut up in an attempt to fit with the stock rings. I superglued the pads to the new, 3d printed rings, and they work well so far. The thickness of the pads and the fact that they are made out of pleather affects the sound signature quite a bit, but the point is that the adapters work.