Surprised by packing on a rudder shaft???? That's... surprising! It's certainly not an unusual situation, LOTS of boats have it. Mixing up imperial and metric sizes are good for lots, and lots of problems.
Guess those guys didn't see too many sailboats... makes me wonder if they actually knew how to cut and fit the packing into the gland... there are an awful lot more wrong ways to do it than right ones.
Pretty much any decent chandlery will have the adjustable packing nut wrench. A good hardware store will have it in the plumbing section for nuts on sink and toilet drains. Plumbers call them "slip wrenches." It is thin enough and short handled enough it does fit on our rudder gland. I am not usually a fan of adjustable wrenches, but if you have to tighten this nut so tight that becomes an issue, the gland needs repacking.
Great Guana Cay, Abacos, Bahamas