Just thought of something...
The threads on male chainring bolts are M8 x 0.75. Again, assuming it will fit...(the threads on Ian's pictured bolt appear to have a much coarser pitch; the A2-70 indicates it is ASTM-302/18-8 stainless, so not marine grade/A4/316) this would extend the socket and allow continuous drainage while shielding the hole from water thrown up from the front wheel.
Best of both worlds.
A stainless one would fill the bill nicely. It would also allow a quick check on size to see if it matches the threads. A standard 5mm allen key will fit for easy installation,
Best,
Dan. (...who sends greetings from very wet and currently flooding Oregon)