With a Preston box on a shuttle, the footplate would be extended then the box placed on the bracket at the front of the shuttle facing backwards.Because the footplate has no support when pushed in (and I thought it might suffer if the weight of the box was on the back of the footplate) I decided to put it on the Preston shuttle with footplate extended. But I did have to make adjustments - I have the rear legs on a wooden platform (at the handle end) and the footplate rests on a wooden block to lift it so it's fairly level. I then put the carryall across the footplate.
From memoryif placed directly onto the shuttle the legs would have been low and would have dragged against the rear wheels - but mine has had a golf trolley motor fixed on the back and the wheels extend outwards more than a basic shuttle. So I'm not sure whether the box just placed on the shuttle would be OK - I think it probably would be. Sorry I can't be any more helpful.