Headstock Top Nut Missing - Urgent Help Or Advice Needed

Hi. It's Saturday night. I'm on a long tour in Northern BC, Yukon, Alaska and just pulled into my night's stay (Pink Mountain, BC, a long way from any major center) to find the very top yoke nut missing.

See attached picture. This is the very top nut that holds down the handlebar assembly.

This is on a 2018 Triumph Tiger 1200 xRT. The pinch bolts are all tight so I think I can carefully ride to, say, Whitehorse on Monday and try to get a fix in place.

Anyone know what diameter and thread is needed? Or whether a part from a Yamaha or other make fits here?

 

Here's the part: https://www.adeptpowersports.com/oem-parts/triumph-nut-upper-yoke-m27-x-1-0-t2040442-part.html

I've figured out from trip photos it's been missing for between 1,600 and 3,000 miles. Motorcycle shops are closed Sunday and Monday so I'm going to see if a machine shop in Whitehorse, YT, 650 miles away, can fabricate one.