I can report a complete success!
I went to Harbor Freight as was suggested. I took the nut with me so I could learn its actual size. I was able to determine it is 26mm...but the only way to get a 26mm wrench at HF was to buy a 10 pack of huge wrenches for $40. I wasn't interested in that. So I looked on Amazon, and found a ratcheted box wrench for $20 that I figured would help me. And THEN....I scrolled down a little bit, and there was this little beauty:
It was only $10 bucks and was for removing the battery on some Dynabrade somethingorother...who cares...it's 26 mm, $10, and just 6" long. Perfect!
I got it and finally got out to the tractor to tighten the safety switch. I turned the key to On so I could hear the relay click on and off as I moved the treadle peddle, to make sure the safety switch was positioned correctly, and then this wrench was just the perfect length to fit in that area and tighten it good. I used a small amount of thread locker on the nut too, hopefully that slows its progress of loosening again. This little wrench now goes into the tractor-specific toolbox....
Thanks for all the help all.