I finally got round to fitting the trim pieces I got from eBay, but they don't fit.
I think I'm using RealOEM correctly. The part that it showed for my 3.0 is 82120151088.
https://www.realoem.com/bmw/enUS/showparts?id=BT52-EUR-02-2004-E85-BMW-Z4_30i&diagId=18_0511
This is also the part number that I purchased from eBay, but they look more round than oval. I didn't want to force them as I may need to return them.
I'm not sure if I just need to man-handle them a bit to get them to fit.