couple options...
search for roll bar clamps on ebay in the size you have above...
do what i did... u-bolts and some aluminum flat bar...
if you oversize the u-bolt, and pad the roof rack with foam tape, you can put a larger piece of flatbar (instead of the included little bar that comes with a u-bolt), drill it, and use 2 quick fists (one on each stud, stacked) to make a quick, easy shovel/axe setup like this...
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