Prodisc C was the disc selected for me. There e over 30 types of discs although there are probably favourites that are used most often. They need to fit hence the custom selection.
A slot is made in the bone and the disc slides into the slot. There isn't, nor can there be, spaces. The discs need to fit snugly. There are no screws except during the surgery as part of the process of implanting them correctly. Your body will produce new bone which will grow around the implant. The surgery is done via the front of the neck which is how you access and remove the damaged disc. Think about it, leaving any kind of space jeopardize the security of the disc. It isn't intended to slide around between the vertebra. You can actually Google an animation of the surgery which is very enlightening.