I had an old pair of sennheiser HD437 laying around i didn't use cause the factory earpads were very thin, which made it unbearable to use for longer periods.
Fortunately i had an extra pair of earcups from another SteelSeries headset, so i made this adapter plate.
The passive open backed HD437 paired up with some good earcups results in rather amazingly good and very light wired headset
I dont know which other sennheiser headsets it would fit, but it should fit most "normal" sized earcups.

Attached is both a slotted and unslotted version, the slotted version keeps the original open backed feature of the headset. The unslotted version is an attempt to make them more like a closed back headset to keep the sound leakage to a minimum.