Thank you for the props, Rich S., John B. and Craig! 
For the top grills, I positioned the (already-pretty-snug-fitting) screen in place in the opening flush with the shell surface, put a tiny drop of CA on the end of .008 wire and carefully applied it to the seam in a few different spots. If the droplet strayed a little onto the finished surface (I applied the screens after painting), I knocked down the buildup and sheen with some ultra-fine modeling sandpaper and touched-up the paint if needed. With the droplets being so tiny, the mess was limited if I miss-hit the target. There's not a lot of CA holding these but, because I spent a lot of time making sure that the screens were a relatively snug fit in the opening to start with and there's no edges to catch, they seem to be pretty darn secure. It also helps that the Eduard mesh is really sturdy stuff and was virtually a perfect size for this project, in that I had a long edge of screen that I could CA along the inward-facing edges of the shell opening (well, at least two opposing edges anyway). The front grill is held in place by pressure alone from wrapping around the side edges of the protruding frame. The screen was bent on both sides so it could be slid over the frame. Getting the cuts and bends so it was a snug fit was no an easy task! 
Ha, you know, until this, it didn't even dawn on me how beautiful those radiator grills are.
They're true works of art Russ.
