On my old Hilux, I drilled two holes through the top bar of my bullbar and ran bolts through through the bar and the jack.

It took a bit of trial and error (noise, vibration, shearing off the bolts) until I got it just right.

The order off 'things' on the bolt (going up) are;

Shaped washer (rounded to the curve of the bullbar)
The bullbar
Shaped washer (as above)
Nylock nut (holds the bolt in place)
Old rubber shackle bush (fits over the nyloc nut and provides shock absorbsion)
Flat washer (same size as rubber bush)
Hi-lift Jack
Flat plate (one long and curved to hold the jack handle in place)
Wingnut.

For security, drill a hole through the bolt above the wingnut and put a padlock through the hole.