Sennheiser hd518 Bluetooth headset mod

I have had my headphones for a while and love the sound and comfort, but i am always disappointed with these features on all the headsets in my price range. ($20-30) I have decided to convert my headphones using a portable Bluetooth receiver I have lying around and a bendy 3.5mm mic. The plan is to mount the receiver to the headphones with Velcro then use a 3.5mm splitter to connect to the headphones and mic. I am however struggling to source an adapter between sennheiser's removable cable jack and 3.5mm as while unofficial adapters should fit the locking system stops them from fitting.

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