Alright, I found one of the write ups, but all it is metal gutter bottom, and probably need some way of holding it to the bumper hole. The grillcraft one has mounting brackets around the edge, but the grill's also thick enough that the brackets were welded onto it.
Here are a few I found that might help give an idea how to do it, since it doesn't look like you'll be able to buy a grill that will work.
And a little more than halfway down on this page
, 2point0 is talking about how he did his lower grill.