For what it's worth, here's another solution to mounting that same rack.
I cut some spacers from steel tubing, somewhere around 2 cm long, and used longer bolts. I know it's not a great solution, but it's good enough that I haven't had to think about it since mounting it. Maybe I should take it apart and send it in for gugification.
You know, this exact solution occurred to me as well - admittedly, well
after the point in time that my rack got shipped off to be modified. It's good to know it actually works though. It's not unusual for my ideas to seem really sound in the back of my mind but to sometimes go a bit south in actual practice!