A 'sometimes' grill block for the Mini should be a piece of cake, if the newer cars are anything like the old (60's) cars. You can get "grill buttons" which make the removal/installation of the grill a snap. Use the stock grille for times when cooling is key, and a plate of aluminum cut to fit at other times. The grill buttons will make changing grilles a sub-minute task.
CLASSIC MINI STAINLESS QUICK RELEASE GRILL BUTTONS (03/29/2011)