You could go with one of those GT3-style bumpers that some people have used on their Boxsters. From what I understand it's not just for looks (though that's usually why people buy them) and actually will provide some additional downforce to the front end.
I'm not sure how much those cup bumpers run new or used, but depending on how much it is, there's always the AeroKit.
Not exactly the kind of dressing one should be using for a dedicated track car, but again, known to actually improve the car's downforce and not meant to be used without a permanent spoiler out back.