i just used a small rivet up top, i had a small screw in it for a while then i put the rivet in for smoother look. Mine seemed to stay for the first week or so without any glue/screws, now it will fall without it. I think some people used silicone or a strong glue, to hold the new pillar to the stock pillar.
The fact that your stock one is falling off, i dont know how to help. I tried just the aftermarket one and it didnt fit right without the stock one in place. Maybe screw the stock one in, then glue/tape the new one to that, if you don't mind putting a hole in your stock one.