Just a tip, Stormforce covers are supposed to be z3 specific and are a popular choice. I found it to not fit at all well, very tight and only reaching mid wheel...i didn't think the material was that great either, I managed to tear mine really easily.
I'm not sure what else is out there, I've got a facelift car so that could have made a difference I suppose. Must be something better in that price range. Anyone?