I got some time to work on it today. I cut another layer of Coroplast and tried to laminate it with hot melt glue. I used spring clamps to hold it to the curved piece until the glue hardens but it failed a few minutes after I removed the clamps.
So, I made some hairpin-like bends out of the wire frame material and stuck them in the ends to hold the two pieces together. Works great, and a couple hammer taps set them deep so they don't stick out of the edge.
At this point, I'd like to take a moment to go off on a tangent rant. Lots of people hate cheap tools as found at discount stores, Harbor Freight, Cummins Tool, etc...but I really got this done a whole lot easier and faster for having lots of cheap specialty tools instead of just having an expensive set of common tools. Heavy duty multi-purpose fence pliers are great for this, and having a couple sizes of bolt cutters is nice because I've got just the right size to do this efficiently. I could have struggled with Snap-On vise grips and electricians pliers but this was a much faster and more pleasant job.
Ok, I digress. I got the second layer laminated on, and the third layer (which I expect to be the visible layer) patterned much more neatly than the previous layers -- I even sacrificed a piece of material just for making the pattern. I made a whole extra set of "hairpins" for attaching them, too.
It's getting less ugly. We have a nice 60 degree November day (I love that global warming!) and I took the day off because I had a rare chance to (usually I just can't even though I've got loads of vacation time built up).
Test-fitting what I think are the final cuts:
I'm now putting coats of Plasti-Dip spray on them and then maybe tomorrow or this weekend I'll be able to mount them. They fit pretty tight, so I've decided that I'll pad any empty space with styrofoam (as I thought before) and then just face-screw them into the grille. That ought to be good enough if I place my screws neatly.
I could also wrap the edges with some trim, such as door edge protectors, but I think it will be acceptable without doing that.
I also entirely removed the grille assembly from the truck and now I'll be able to work on that indoors when it's cold and dark out.