The factory supplied cover has a slot horizontally cut below the rib near the bottom before chrome plating. If you want to diy the mod I would advise a series of drilled holes which will leave a better support for the chrome plating which might start to peel from the long edge of a slot. See mine here http://www.bleazey.co.uk/M3W.html

Phil

2013 M3W 1960 Velocette Venom and the Landrovardo!