hm. what annoys me even more is that the rack is not even a perfect fit, its slightly thinner than what it should be (and thus wobbles a bit..)

I'm still baffled by all the pictures I'm seeing from people riding with heavy adult passengers on the back; how do they secure the rack so well?? I can't imagine having so much weight on there with a simple snap deck that pushes the two racks apart. The downwards force is not so much of a problem, its the sideways force I'm worried about.
I guess what I can do is add those double seat clamps, maybe that'll improve things. And then tie the two racks together to get some inward tension going. I'm also tempted to push an extra aluminum tube inside the racks just to make them stiffer at the bottom (where all the side force will apply).
Do the bags they sell at xtracycle somehow add some strength too? I didn't get them.