Well this thread got me motivated enough to finally get around to making my jack mount.
I kept it simple and cheep. I used 2” square tube, drilled two holes, and welded nuts inside to bolt the mounts to the top of the trailer fender. I welded a bolt to the inside of the end cap to hold the jack to the mount. I used lugnuts off an old Ford truck to retain the jack onto the mounts. I wanted it so that when I had the truck parked at a store, restaurant or whatever unattended, that someone couldn’t just walk up, undo a latch and run away with my jack. With the lugnuts tight, they would have to have the proper wrench on hand, and it still would take more than a few seconds to pull it off. It’s not fool proof, but give just that little bit more than just a strap or wing nut would provided. And if I do require to use the jack, so what if it takes me a minute longer to take it off the fender. And yes, I could use anti-theft lugnuts that require the special key socket, but I'm not that worried about it.