Having venting or holes in your rig is typically not a big deal 99% of the time.
To install them simply put them in place and use a screw on the top part to hold it there. Notice that you will need a PCI cover, and not a bend-to-remove slug for this to work. If you really want it covered then just about any kind of tape (preferably metal/foil ducting tape) will do the trick.
As a general rule of thumb, if your computer is in a location that is fairly dust free then it does not matter if your case is perfectly airtight. Personally I run a high-end system with very low air flow (due to noise concerns), and so I have concerns about having airflow directed at specific places and taking a certin route within my computer, so I have all...