After looking at various available attachments and closure clips for balloons, I decided that the easiest and best thing to use was a simple cable tie. I did have a video of how this would work, but I've managed to loose it somehow.
But regardless, a cable tie can be pulled easily and will close the balloon securely. This form of mechanism that I have developed would allow the balloon attach to the camera easily without causing any extra work for the user, trying to keep this as simple as possible.
Hopefully the cable tie would be long enough so that the user can pull it easily without having to fiddle about underneath the giant inflated balloon.
Initial thoughts, if this method is carried forward, is that the cable tie would be pre tied around the balloon and cased in the little viewer like box. This could then be purchased seperately enabling the camera to be reused (although I would like to look into what disposable technology is and if this is relevent to my project.)
This is just an initial thought and is still a part of the project that I'm continuing to play with.