you want something that is thin and very sticky.
Amazon.com: 3m double sided adhesive: Electronics
I use carbon tape from work that is used for Scanning Electron Microscopy, but 3M double sided adhesive tape at Stapes etc. will do a decent job as well. It is a little thicker compared to the adhesive in the link above and the camera cover may not be completely flush with the housing.
Since the camera cover was missing, I am wondering if the lens cover (fits into the hole in the camera cover you had to buy) is also missing. That basically protects the actual camera inside the phone and ensures no dust gets into the phone. It may also be still attached to the frame inside the phone.
see attached images.
you see all three parts in this image, the black part that shows in the yellow housing towards the top of where the camera cover goes is inside the phone (it is silver on the other side of the housing, i.e. inside). The small lens cover out of glass that fits into the camera cover and the camera cover itself.
this shows the small round lens cover inside the camera cover. it should be flush towards the outside.