Now this is a classic, from the reference to a 25mm threaded port, to the fact that the port in question is referred to as an ‘ingold’. In this case, one classic solution deserves another. I would recommend the Keofitt sampling valve, which is fast approaching its 40th birthday and still going strong with 350,000+ valves rumored to be in operation. Also, since you did not mention if the port on your vessel is the standard or safety design, be sure to double check as this will determine which Keofitt valve length you need for a correct o-ring placement.

Keofitt