Be careful about gluing it too much. It should only be glued on the edges and around the open holes. When the sun hits it and your windows are rolled up, that cheap plastic starts expanding and shrinking.
That's funny that yours has perforated speaker holes. Are they right over the original speakers? They look like they are too far too the center. I have a cap where the CENTER SPEAKER is just perforated holes, but the outside speakers, I was able to cut the holes and install the 3 speaker grills.
I use the silicone that comes with the caps. I use those binder clips that you get at the office supply store or in your office's supply cabinet.