D2 Radiator cap pressure relief

Lost the radiator cap to my D2 the last 53 inch wet snow in the spring. After looking all summer long on the mountain and on the web I decided to build one myself.
I am wondering if anyone knows what the coolant system pressure on a D2 might be? I figure I better make a pressure relief maybe with a ball bearing and a spring and will probably end up doing it trial and error anyway but thought I'd ask for ideas. Thanks, Steve.