Wait, I just had a better thought for securing the rack in case of hinge failure. Instead of ubolts, drill two vertical holes through both horizontal bars (closer to the hinges the better) and use a bolt and cotter pin through each one. This won't do much for your rattles but it will keep the top bar on if/when the hinges break. To make the hinges last longer, I'd look at using wedges or stops (cutting board material works great) to minimize the up,down, fore, and aft movement.
For the rattles, you could drill a horizontal hole through each top bar close to each end (the end that's closest to the middle of the truck). Behind these holes have the welding shop weld a vertical piece of steel with a nut welded to it (welded to the bottom bar). Then have a bolt with handle that goes through the horizontal hole in the top bar and screws into the piece of steel with the welded nut. I'd also have them weld a washer to the bolt so that the T part of the handle doesn't bottom out against either bar and scratch it up. If you wanted to keep it simple, you could just use a bolt without a handle and buy one of those harbor freight ratcheting wrenches and keep it safety wired to the rack.
In case that description isn't clear, the idea behind it is that it allows the top bar to be securely tightened into the bottom bar, but on a horizontal, and not vertical plane. This will have the same effect as moving your toggle clamp but should be easier and cheaper to do. Plus, the distance between the two swingouts won't matter so he/she won't have to recut or reposition anything.
To open the swingout, you'd have to spend 10 seconds pulling the pins and another 5-10 seconds undoing each bolt. All in all about 30 seconds of work but I think it'd be worth it for peace of mind as far as safety goes, and peace of travel as far as lessening squeaks and rattles goes.