IMO, the easiest way is to take a drill with a significantly larger bit than the welds and drill out the few spot welds holding it in. You will have a few small holes if you go all the way through the welds, but if you're careful and have access to a flat-nosed (I think the technical term is a bottoming bit?) bit you can easily get just the welds.
I cannot remember how many welds there are, but you can see them and there aren't too many, they look like dimples in the surface of the tray. There are a few on top, one on the tab that extends up the fender well, and 2 or 3 on the support that extends down to the frame rail area.
I can get pictures of my rust bucket's tray and circle them if you have trouble with it! Good luck!
