as I've posted in many previous threads, we also have an iSi rack.
Initially it was fitted to the back of our 105 series and in that time it crossed the Simpson twice, the Anne Beadel once and more smaller tracks and outback trips than I can count. Not a single problem.
And unlike many rear of car mounted racks, not one piece of plastic in the construction and most importantly (to me at least), the rack doesn't impede the approach or departure angles of the car.
We now have it fitted to the back of the "half an 80 series" camper (in the tow hitch receiver) and it's been perfect. Up high, out of the way and tougher than many 4wd accessories...
Also, if you really want a custom mount, George at iSi is more than willing to talk to you and help with custom designs to suit different campers if required.