Coolant refill problem solver:
IIRC ,when refilling it is easy to get big air-lock problems.
The smart tool to use is a Vacuum filling kit - UView Airlift 550.000 is the Vacuum tool often mentioned but it is expensive for a seldom used tool - $130+.
Amazon.com: UView 550000 Airlift Cooling System Leak Checker and Airlock Purge Tool Kit: Automotive
I found a half price generic version and wonder if anyone has tried it? It is approx. $72 on Fleabay and other sources.
Amazon.com: FJC 43610 Radiator Coolant Vacuum Refill Kit: Automotive
There are many vendors of this tool # 43610 so it may be re-branded.
I have tried a Lisle funnel on my modified Miata with mediocre results:
Amazon.com: Lisle 24680 Spill-Free Funnel: Automotive
This may help on the Boxster if you raise the rear of the car very high?