Dear BIGUN,
Thanks for the recommendation ... but it has been more than a decade since I sewed a replacement cosmetic cap for a Reflex. Actually, it is more than a cosmetic cap because it protects the Cypres loop.
I started by laying a tape measure over the fiberglass cap, then adding a little (0.12 or 0.5 inches) for the binding tape.
I forget how many layers of fabric are in a Reflex cap: Cordura, foam, ballistic and ? I would need to look at an original to confirm the number of layers. I sewed them together around the circumference, but allowed thread tension to shrink the perimeter. Then I sewed on the channel for the elastic cord.
Then I bound the edge and finished binding with the same trick as Javelin pilot chute caps: diagonal end ....
The last step was installing the bungee cord (cloth wrapped around rubber for North Americans). I used the same style of bungee cord as is used in free-bags. My first bungee cord was too long and it blew off in freefall, so I made the second cap with an even shorter bungee cord. The second cap lasted a few years.