which do you mean, the angled one or the vertical one?
I mounted mine on the top of the angled one by putting a piece of threaded rod through the lower hole meant for the spare tire holder, putting locknuts on the rod on either side of the rollbar to rigidly mount it.
Then I used another locknut and washer as a lower stop, slide one of the holes in the jack over the threaded rod and then use a short piece of heater hose, another washer and locknut to tighten down a little on the jack.
I store a couple of rags under the foot of the jack to keep the bottom from rattling around and use a bungee cord around the jack and rollbar right near the bottom to hold that end in place. I also put a hose clamp around the "bar" and handle to keep the handle from rattling.
Cost all of about $4 and you'd never know it was there... not one rattle.
Look on the left side of this pic (I was using duct tape on the handle at the time)...
There's also another thread on this
here