I had the same thing happen. I took some epoxy and mixed in a little bit of cut up steel wool after I really roughed up the mating surfaces. I spread the epoxy around on both mating surfaces, stuck the parts together, then put the remainder of the epoxy on the inner surface of the back part of the center cap - the area you have showing in the photo. After I let it set up a week, I tried to get the two parts apart by hand, and had no luck doing so.
If I worked REALLY hard, I'm sure I could get them apart. But that kind of force is not seen by the cap under regular use. And if I want the caps out of the wheels, I'll just have to remember to take the wheel off to push the cap out from the back.