Im the same - always need a window open slightly, regardless of the weather!
As others have said, theres sod all you can do about it really, unless you try to get hold of one of these:
http://www.weathertecheurope.com/assets/21/31/GalleryEnlarged4DimensionId/SWD_CU_Jill_2415.JPG
Cant find any that would fit a ragtop Z4 though!