It looks like the B is not the only current car with no holder. I did an Amazon search and I discovered a Mercedes specific holder. I ordered one and it should be here on wednesday.
https://www.amazon.com/gp/product/B01HI2R0LO/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1#
https://www.amazon.com/gp/product/B01HI2R0LO/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1#