I understand what you are saying.
I don't know much about the Mugen but since you got the one piece tray did you make sure you had 1.25" clearance from the spur to the mud guard before ordering the tray?
That's the clearance that is needed so it clears the spur and Mike states this when you go and order the one piece tray .

With mounting it drill your own holes as most of us do, since the holes in the chassis rarely line up in a good mounting location.