Holy cow, that's ridiculous! I can't believe you would weld lights to your hatch when there are perfectly good alternatives. You said yourself you saw the U-bolt mounting options, what makes you think that wouldn't be secure enough? I can't find the pictures of Ramness's setup, but there's a similar pic on Coolerman's site:

That's well below the top of the rack, and more than secure enough for a 6 oz. light.