I tried one of the store bought locks as pictured in post #3, but took it back because I couldn't get it to work right. YMMV!
Here is a picture of what I made for my Ford 8N. Cost to make was about $5. I no longer use it because I bought a KK trailer mover to use instead.
View attachment 336643
You are also going to need stay strap or the chain the draw bar to keep it from going up.