When I owned my 911 I use to use these guys alot, so this is the one I ordered.
Porsche 991 Water Tank Reservoir Cap 99110645701 - 99110645701 | Design 911
Look lovely, just ordered the coolant cap last week from Fleabay but it's taking an age to arrive. I've also got R8 oil cap but R8 coolant cap was too steep for my budget!