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Microphone case. Music shop might just be able to give you one they have lying about. I got one for £2 on eBay.
What about the official softcase from Unit Portables?
Thanks for all the responses. I ordered one of these off Sennheiser in the end for £25. Someone testified over on MuffWiggler to it being a decent fit. Looks alright too.https://www.bhphotovideo.com/c/product/426802-REG/Sennheiser_CASEMD441_CASEMD441_Hard_Case.html