When installing a HDD instead of the SuperDrive:
Does the bracket, which holds the HDD, seal off the optical drive slot in the unibody?
If not, it seems that debris and dust could get in. (I guess it may depend as there are various types of bracket.)
But if it does get sealed, I wonder if closing that gap can negatively affect airflow inside the machine, and thus, risk overheating.
Also, perhaps those bristles can be removed, and an aluminum/plastic filling could close it up? Something that would be flush with the outside. Is there something like that?